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Re: [PATCH v2 0/3] virtio: increase VIRTQUEUE_MAX_SIZE to 32k

From: Stefan Hajnoczi
Subject: Re: [PATCH v2 0/3] virtio: increase VIRTQUEUE_MAX_SIZE to 32k
Date: Wed, 3 Nov 2021 11:33:33 +0000 
On Mon, Nov 01, 2021 at 09:29:26PM +0100, Christian Schoenebeck wrote:
> On Donnerstag, 28. Oktober 2021 11:00:48 CET Stefan Hajnoczi wrote:
> > On Mon, Oct 25, 2021 at 05:03:25PM +0200, Christian Schoenebeck wrote:
> > > On Montag, 25. Oktober 2021 12:30:41 CEST Stefan Hajnoczi wrote:
> > > > On Thu, Oct 21, 2021 at 05:39:28PM +0200, Christian Schoenebeck wrote:
> > > > > On Freitag, 8. Oktober 2021 18:08:48 CEST Christian Schoenebeck wrote:
> > > > > > On Freitag, 8. Oktober 2021 16:24:42 CEST Christian Schoenebeck 
> > > > > > wrote:
> > > > > > > On Freitag, 8. Oktober 2021 09:25:33 CEST Greg Kurz wrote:
> > > > > > > > On Thu, 7 Oct 2021 16:42:49 +0100
> > > > > > > > 
> > > > > > > > Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > > > > > > > > On Thu, Oct 07, 2021 at 02:51:55PM +0200, Christian 
> > > > > > > > > Schoenebeck wrote:
> > > > > > > > > > On Donnerstag, 7. Oktober 2021 07:23:59 CEST Stefan 
> > > > > > > > > > Hajnoczi wrote:
> > > > > > > > > > > On Mon, Oct 04, 2021 at 09:38:00PM +0200, Christian
> > > > > > > > > > > Schoenebeck
> > > > > > 
> > > > > > wrote:
> > > > > > > > > > > > At the moment the maximum transfer size with virtio is
> > > > > > > > > > > > limited
> > > > > > > > > > > > to
> > > > > > > > > > > > 4M
> > > > > > > > > > > > (1024 * PAGE_SIZE). This series raises this limit to its
> > > > > > > > > > > > maximum
> > > > > > > > > > > > theoretical possible transfer size of 128M (32k pages)
> > > > > > > > > > > > according
> > > > > > > > > > > > to
> > > > > > > > > > > > the
> > > > > > > > > > > > virtio specs:
> > > > > > > > > > > > 
> > > > > > > > > > > > https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virt
> > > > > > > > > > > > io-v
> > > > > > > > > > > > 1.1-
> > > > > > > > > > > > cs
> > > > > > > > > > > > 01
> > > > > > > > > > > > .html#
> > > > > > > > > > > > x1-240006
> > > > > > > > > > > 
> > > > > > > > > > > Hi Christian,
> > > > > > > > 
> > > > > > > > > > > I took a quick look at the code:
> > > > > > > > Hi,
> > > > > > > > 
> > > > > > > > Thanks Stefan for sharing virtio expertise and helping Christian
> > > > > > > > !
> > > > > > > > 
> > > > > > > > > > > - The Linux 9p driver restricts descriptor chains to 128
> > > > > > > > > > > elements
> > > > > > > > > > > 
> > > > > > > > > > >   (net/9p/trans_virtio.c:VIRTQUEUE_NUM)
> > > > > > > > > > 
> > > > > > > > > > Yes, that's the limitation that I am about to remove (WIP);
> > > > > > > > > > current
> > > > > > > > > > kernel
> > > > > > > > > > patches:
> > > > > > > > > > https://lore.kernel.org/netdev/cover.1632327421.git.linux_os
> > > > > > > > > > s@cr
> > > > > > > > > > udeb
> > > > > > > > > > yt
> > > > > > > > > > e.
> > > > > > > > > > com/>
> > > > > > > > > 
> > > > > > > > > I haven't read the patches yet but I'm concerned that today
> > > > > > > > > the
> > > > > > > > > driver
> > > > > > > > > is pretty well-behaved and this new patch series introduces a
> > > > > > > > > spec
> > > > > > > > > violation. Not fixing existing spec violations is okay, but
> > > > > > > > > adding
> > > > > > > > > new
> > > > > > > > > ones is a red flag. I think we need to figure out a clean
> > > > > > > > > solution.
> > > > > > > 
> > > > > > > Nobody has reviewed the kernel patches yet. My main concern
> > > > > > > therefore
> > > > > > > actually is that the kernel patches are already too complex,
> > > > > > > because
> > > > > > > the
> > > > > > > current situation is that only Dominique is handling 9p patches on
> > > > > > > kernel
> > > > > > > side, and he barely has time for 9p anymore.
> > > > > > > 
> > > > > > > Another reason for me to catch up on reading current kernel code
> > > > > > > and
> > > > > > > stepping in as reviewer of 9p on kernel side ASAP, independent of
> > > > > > > this
> > > > > > > issue.
> > > > > > > 
> > > > > > > As for current kernel patches' complexity: I can certainly drop
> > > > > > > patch
> > > > > > > 7
> > > > > > > entirely as it is probably just overkill. Patch 4 is then the
> > > > > > > biggest
> > > > > > > chunk, I have to see if I can simplify it, and whether it would
> > > > > > > make
> > > > > > > sense to squash with patch 3.
> > > > > > > 
> > > > > > > > > > > - The QEMU 9pfs code passes iovecs directly to preadv(2)
> > > > > > > > > > > and
> > > > > > > > > > > will
> > > > > > > > > > > fail
> > > > > > > > > > > 
> > > > > > > > > > >   with EINVAL when called with more than IOV_MAX iovecs
> > > > > > > > > > >   (hw/9pfs/9p.c:v9fs_read())
> > > > > > > > > > 
> > > > > > > > > > Hmm, which makes me wonder why I never encountered this
> > > > > > > > > > error
> > > > > > > > > > during
> > > > > > > > > > testing.
> > > > > > > > > > 
> > > > > > > > > > Most people will use the 9p qemu 'local' fs driver backend
> > > > > > > > > > in
> > > > > > > > > > practice,
> > > > > > > > > > so
> > > > > > > > > > that v9fs_read() call would translate for most people to
> > > > > > > > > > this
> > > > > > > > > > implementation on QEMU side (hw/9p/9p-local.c):
> > > > > > > > > > 
> > > > > > > > > > static ssize_t local_preadv(FsContext *ctx, V9fsFidOpenState
> > > > > > > > > > *fs,
> > > > > > > > > > 
> > > > > > > > > >                             const struct iovec *iov,
> > > > > > > > > >                             int iovcnt, off_t offset)
> > > > > > > > > > 
> > > > > > > > > > {
> > > > > > > > > > #ifdef CONFIG_PREADV
> > > > > > > > > > 
> > > > > > > > > >     return preadv(fs->fd, iov, iovcnt, offset);
> > > > > > > > > > 
> > > > > > > > > > #else
> > > > > > > > > > 
> > > > > > > > > >     int err = lseek(fs->fd, offset, SEEK_SET);
> > > > > > > > > >     if (err == -1) {
> > > > > > > > > >     
> > > > > > > > > >         return err;
> > > > > > > > > >     
> > > > > > > > > >     } else {
> > > > > > > > > >     
> > > > > > > > > >         return readv(fs->fd, iov, iovcnt);
> > > > > > > > > >     
> > > > > > > > > >     }
> > > > > > > > > > 
> > > > > > > > > > #endif
> > > > > > > > > > }
> > > > > > > > > > 
> > > > > > > > > > > Unless I misunderstood the code, neither side can take
> > > > > > > > > > > advantage
> > > > > > > > > > > of
> > > > > > > > > > > the
> > > > > > > > > > > new 32k descriptor chain limit?
> > > > > > > > > > > 
> > > > > > > > > > > Thanks,
> > > > > > > > > > > Stefan
> > > > > > > > > > 
> > > > > > > > > > I need to check that when I have some more time. One
> > > > > > > > > > possible
> > > > > > > > > > explanation
> > > > > > > > > > might be that preadv() already has this wrapped into a loop
> > > > > > > > > > in
> > > > > > > > > > its
> > > > > > > > > > implementation to circumvent a limit like IOV_MAX. It might
> > > > > > > > > > be
> > > > > > > > > > another
> > > > > > > > > > "it
> > > > > > > > > > works, but not portable" issue, but not sure.
> > > > > > > > > > 
> > > > > > > > > > There are still a bunch of other issues I have to resolve.
> > > > > > > > > > If
> > > > > > > > > > you
> > > > > > > > > > look
> > > > > > > > > > at
> > > > > > > > > > net/9p/client.c on kernel side, you'll notice that it
> > > > > > > > > > basically
> > > > > > > > > > does
> > > > > > > > > > this ATM> >
> > > > > > > > > > 
> > > > > > > > > >     kmalloc(msize);
> > > > > > > > 
> > > > > > > > Note that this is done twice : once for the T message (client
> > > > > > > > request)
> > > > > > > > and
> > > > > > > > once for the R message (server answer). The 9p driver could
> > > > > > > > adjust
> > > > > > > > the
> > > > > > > > size
> > > > > > > > of the T message to what's really needed instead of allocating
> > > > > > > > the
> > > > > > > > full
> > > > > > > > msize. R message size is not known though.
> > > > > > > 
> > > > > > > Would it make sense adding a second virtio ring, dedicated to
> > > > > > > server
> > > > > > > responses to solve this? IIRC 9p server already calculates
> > > > > > > appropriate
> > > > > > > exact sizes for each response type. So server could just push
> > > > > > > space
> > > > > > > that's
> > > > > > > really needed for its responses.
> > > > > > > 
> > > > > > > > > > for every 9p request. So not only does it allocate much more
> > > > > > > > > > memory
> > > > > > > > > > for
> > > > > > > > > > every request than actually required (i.e. say 9pfs was
> > > > > > > > > > mounted
> > > > > > > > > > with
> > > > > > > > > > msize=8M, then a 9p request that actually would just need 1k
> > > > > > > > > > would
> > > > > > > > > > nevertheless allocate 8M), but also it allocates >
> > > > > > > > > > PAGE_SIZE,
> > > > > > > > > > which
> > > > > > > > > > obviously may fail at any time.>
> > > > > > > > > 
> > > > > > > > > The PAGE_SIZE limitation sounds like a kmalloc() vs vmalloc()
> > > > > > > > > situation.
> > > > > > > 
> > > > > > > Hu, I didn't even consider vmalloc(). I just tried the kvmalloc()
> > > > > > > wrapper
> > > > > > > as a quick & dirty test, but it crashed in the same way as
> > > > > > > kmalloc()
> > > > > > > with
> > > > > > > large msize values immediately on mounting:
> > > > > > > 
> > > > > > > diff --git a/net/9p/client.c b/net/9p/client.c
> > > > > > > index a75034fa249b..cfe300a4b6ca 100644
> > > > > > > --- a/net/9p/client.c
> > > > > > > +++ b/net/9p/client.c
> > > > > > > @@ -227,15 +227,18 @@ static int parse_opts(char *opts, struct
> > > > > > > p9_client
> > > > > > > *clnt)
> > > > > > > 
> > > > > > >  static int p9_fcall_init(struct p9_client *c, struct p9_fcall
> > > > > > >  *fc,
> > > > > > >  
> > > > > > >                          int alloc_msize)
> > > > > > >  
> > > > > > >  {
> > > > > > > 
> > > > > > > -       if (likely(c->fcall_cache) && alloc_msize == c->msize) {
> > > > > > > +       //if (likely(c->fcall_cache) && alloc_msize == c->msize) {
> > > > > > > +       if (false) {
> > > > > > > 
> > > > > > >                 fc->sdata = kmem_cache_alloc(c->fcall_cache,
> > > > > > >                 GFP_NOFS);
> > > > > > >                 fc->cache = c->fcall_cache;
> > > > > > >         
> > > > > > >         } else {
> > > > > > > 
> > > > > > > -               fc->sdata = kmalloc(alloc_msize, GFP_NOFS);
> > > > > > > +               fc->sdata = kvmalloc(alloc_msize, GFP_NOFS);
> > > > > > 
> > > > > > Ok, GFP_NOFS -> GFP_KERNEL did the trick.
> > > > > > 
> > > > > > Now I get:
> > > > > >    virtio: bogus descriptor or out of resources
> > > > > > 
> > > > > > So, still some work ahead on both ends.
> > > > > 
> > > > > Few hacks later (only changes on 9p client side) I got this running
> > > > > stable
> > > > > now. The reason for the virtio error above was that kvmalloc() returns
> > > > > a
> > > > > non-logical kernel address for any kvmalloc(>4M), i.e. an address that
> > > > > is
> > > > > inaccessible from host side, hence that "bogus descriptor" message by
> > > > > QEMU.
> > > > > So I had to split those linear 9p client buffers into sparse ones (set
> > > > > of
> > > > > individual pages).
> > > > > 
> > > > > I tested this for some days with various virtio transmission sizes and
> > > > > it
> > > > > works as expected up to 128 MB (more precisely: 128 MB read space +
> > > > > 128 MB
> > > > > write space per virtio round trip message).
> > > > > 
> > > > > I did not encounter a show stopper for large virtio transmission sizes
> > > > > (4 MB ... 128 MB) on virtio level, neither as a result of testing, nor
> > > > > after reviewing the existing code.
> > > > > 
> > > > > About IOV_MAX: that's apparently not an issue on virtio level. Most of
> > > > > the
> > > > > iovec code, both on Linux kernel side and on QEMU side do not have
> > > > > this
> > > > > limitation. It is apparently however indeed a limitation for userland
> > > > > apps
> > > > > calling the Linux kernel's syscalls yet.
> > > > > 
> > > > > Stefan, as it stands now, I am even more convinced that the upper
> > > > > virtio
> > > > > transmission size limit should not be squeezed into the queue size
> > > > > argument of virtio_add_queue(). Not because of the previous argument
> > > > > that
> > > > > it would waste space (~1MB), but rather because they are two different
> > > > > things. To outline this, just a quick recap of what happens exactly
> > > > > when
> > > > > a bulk message is pushed over the virtio wire (assuming virtio "split"
> > > > > layout here):
> > > > > 
> > > > > ---------- [recap-start] ----------
> > > > > 
> > > > > For each bulk message sent guest <-> host, exactly *one* of the
> > > > > pre-allocated descriptors is taken and placed (subsequently) into
> > > > > exactly
> > > > > *one* position of the two available/used ring buffers. The actual
> > > > > descriptor table though, containing all the DMA addresses of the
> > > > > message
> > > > > bulk data, is allocated just in time for each round trip message. Say,
> > > > > it
> > > > > is the first message sent, it yields in the following structure:
> > > > > 
> > > > > Ring Buffer   Descriptor Table      Bulk Data Pages
> > > > > 
> > > > >    +-+              +-+           +-----------------+
> > > > >    
> > > > >    |D|------------->|d|---------->| Bulk data block |
> > > > >    
> > > > >    +-+              |d|--------+  +-----------------+
> > > > >    
> > > > >    | |              |d|------+ |
> > > > >    
> > > > >    +-+               .       | |  +-----------------+
> > > > >    
> > > > >    | |               .       | +->| Bulk data block |
> > > > >     
> > > > >     .                .       |    +-----------------+
> > > > >     .               |d|-+    |
> > > > >     .               +-+ |    |    +-----------------+
> > > > >     
> > > > >    | |                  |    +--->| Bulk data block |
> > > > >    
> > > > >    +-+                  |         +-----------------+
> > > > >    
> > > > >    | |                  |                 .
> > > > >    
> > > > >    +-+                  |                 .
> > > > >    
> > > > >                         |                 .
> > > > >                         |         
> > > > >                         |         +-----------------+
> > > > >                         
> > > > >                         +-------->| Bulk data block |
> > > > >                         
> > > > >                                   +-----------------+
> > > > > 
> > > > > Legend:
> > > > > D: pre-allocated descriptor
> > > > > d: just in time allocated descriptor
> > > > > -->: memory pointer (DMA)
> > > > > 
> > > > > The bulk data blocks are allocated by the respective device driver
> > > > > above
> > > > > virtio subsystem level (guest side).
> > > > > 
> > > > > There are exactly as many descriptors pre-allocated (D) as the size of
> > > > > a
> > > > > ring buffer.
> > > > > 
> > > > > A "descriptor" is more or less just a chainable DMA memory pointer;
> > > > > defined
> > > > > as:
> > > > > 
> > > > > /* Virtio ring descriptors: 16 bytes.  These can chain together via
> > > > > "next". */ struct vring_desc {
> > > > > 
> > > > >       /* Address (guest-physical). */
> > > > >       __virtio64 addr;
> > > > >       /* Length. */
> > > > >       __virtio32 len;
> > > > >       /* The flags as indicated above. */
> > > > >       __virtio16 flags;
> > > > >       /* We chain unused descriptors via this, too */
> > > > >       __virtio16 next;
> > > > > 
> > > > > };
> > > > > 
> > > > > There are 2 ring buffers; the "available" ring buffer is for sending a
> > > > > message guest->host (which will transmit DMA addresses of guest
> > > > > allocated
> > > > > bulk data blocks that are used for data sent to device, and separate
> > > > > guest allocated bulk data blocks that will be used by host side to
> > > > > place
> > > > > its response bulk data), and the "used" ring buffer is for sending
> > > > > host->guest to let guest know about host's response and that it could
> > > > > now
> > > > > safely consume and then deallocate the bulk data blocks subsequently.
> > > > > 
> > > > > ---------- [recap-end] ----------
> > > > > 
> > > > > So the "queue size" actually defines the ringbuffer size. It does not
> > > > > define the maximum amount of descriptors. The "queue size" rather
> > > > > defines
> > > > > how many pending messages can be pushed into either one ringbuffer
> > > > > before
> > > > > the other side would need to wait until the counter side would step up
> > > > > (i.e. ring buffer full).
> > > > > 
> > > > > The maximum amount of descriptors (what VIRTQUEUE_MAX_SIZE actually
> > > > > is)
> > > > > OTOH defines the max. bulk data size that could be transmitted with
> > > > > each
> > > > > virtio round trip message.
> > > > > 
> > > > > And in fact, 9p currently handles the virtio "queue size" as directly
> > > > > associative with its maximum amount of active 9p requests the server
> > > > > could
> > > > > 
> > > > > handle simultaniously:
> > > > >   hw/9pfs/9p.h:#define MAX_REQ         128
> > > > >   hw/9pfs/9p.h:    V9fsPDU pdus[MAX_REQ];
> > > > >   hw/9pfs/virtio-9p-device.c:    v->vq = virtio_add_queue(vdev,
> > > > >   MAX_REQ,
> > > > >   
> > > > >                                  handle_9p_output);
> > > > > 
> > > > > So if I would change it like this, just for the purpose to increase
> > > > > the
> > > > > max. virtio transmission size:
> > > > > 
> > > > > --- a/hw/9pfs/virtio-9p-device.c
> > > > > +++ b/hw/9pfs/virtio-9p-device.c
> > > > > @@ -218,7 +218,7 @@ static void virtio_9p_device_realize(DeviceState
> > > > > *dev,
> > > > > Error **errp)>
> > > > > 
> > > > >      v->config_size = sizeof(struct virtio_9p_config) +
> > > > >      strlen(s->fsconf.tag);
> > > > >      virtio_init(vdev, "virtio-9p", VIRTIO_ID_9P, v->config_size,
> > > > >      
> > > > >                  VIRTQUEUE_MAX_SIZE);
> > > > > 
> > > > > -    v->vq = virtio_add_queue(vdev, MAX_REQ, handle_9p_output);
> > > > > +    v->vq = virtio_add_queue(vdev, 32*1024, handle_9p_output);
> > > > > 
> > > > >  }
> > > > > 
> > > > > Then it would require additional synchronization code on both ends and
> > > > > therefore unnecessary complexity, because it would now be possible
> > > > > that
> > > > > more requests are pushed into the ringbuffer than server could handle.
> > > > > 
> > > > > There is one potential issue though that probably did justify the
> > > > > "don't
> > > > > exceed the queue size" rule:
> > > > > 
> > > > > ATM the descriptor table is allocated (just in time) as *one*
> > > > > continuous
> > > > > buffer via kmalloc_array():
> > > > > https://github.com/torvalds/linux/blob/2f111a6fd5b5297b4e92f53798ca086
> > > > > f7c7
> > > > > d33a4/drivers/virtio/virtio_ring.c#L440
> > > > > 
> > > > > So assuming transmission size of 2 * 128 MB that kmalloc_array() call
> > > > > would
> > > > > yield in kmalloc(1M) and the latter might fail if guest had highly
> > > > > fragmented physical memory. For such kind of error case there is
> > > > > currently a fallback path in virtqueue_add_split() that would then use
> > > > > the required amount of pre-allocated descriptors instead:
> > > > > https://github.com/torvalds/linux/blob/2f111a6fd5b5297b4e92f53798ca086
> > > > > f7c7
> > > > > d33a4/drivers/virtio/virtio_ring.c#L525
> > > > > 
> > > > > That fallback recovery path would no longer be viable if the queue
> > > > > size
> > > > > was
> > > > > exceeded. There would be alternatives though, e.g. by allowing to
> > > > > chain
> > > > > indirect descriptor tables (currently prohibited by the virtio specs).
> > > > 
> > > > Making the maximum number of descriptors independent of the queue size
> > > > requires a change to the VIRTIO spec since the two values are currently
> > > > explicitly tied together by the spec.
> > > 
> > > Yes, that's what the virtio specs say. But they don't say why, nor did I
> > > hear a reason in this dicussion.
> > > 
> > > That's why I invested time reviewing current virtio implementation and
> > > specs, as well as actually testing exceeding that limit. And as I
> > > outlined in detail in my previous email, I only found one theoretical
> > > issue that could be addressed though.
> > 
> > I agree that there is a limitation in the VIRTIO spec, but violating the
> > spec isn't an acceptable solution:
> > 
> > 1. QEMU and Linux aren't the only components that implement VIRTIO. You
> >    cannot make assumptions about their implementations because it may
> >    break spec-compliant implementations that you haven't looked at.
> > 
> >    Your patches weren't able to increase Queue Size because some device
> >    implementations break when descriptor chains are too long. This shows
> >    there is a practical issue even in QEMU.
> > 
> > 2. The specific spec violation that we discussed creates the problem
> >    that drivers can no longer determine the maximum description chain
> >    length. This in turn will lead to more implementation-specific
> >    assumptions being baked into drivers and cause problems with
> >    interoperability and future changes.
> > 
> > The spec needs to be extended instead. I included an idea for how to do
> > that below.
> 
> Sure, I just wanted to see if there was a non-neglectable "hard" show stopper
> per se that I probably haven't seen yet. I have not questioned aiming a clean
> solution.
> 
> Thanks for the clarification!
> 
> > > > Before doing that, are there benchmark results showing that 1 MB vs 128
> > > > MB produces a performance improvement? I'm asking because if performance
> > > > with 1 MB is good then you can probably do that without having to change
> > > > VIRTIO and also because it's counter-intuitive that 9p needs 128 MB for
> > > > good performance when it's ultimately implemented on top of disk and
> > > > network I/O that have lower size limits.
> > > 
> > > First some numbers, linear reading a 12 GB file:
> > > 
> > > msize    average      notes
> > > 
> > > 8 kB     52.0 MB/s    default msize of Linux kernel <v5.15
> > > 128 kB   624.8 MB/s   default msize of Linux kernel >=v5.15
> > > 512 kB   1961 MB/s    current max. msize with any Linux kernel <=v5.15
> > > 1 MB     2551 MB/s    this msize would already violate virtio specs
> > > 2 MB     2521 MB/s    this msize would already violate virtio specs
> > > 4 MB     2628 MB/s    planned max. msize of my current kernel patches [1]
> > 
> > How many descriptors are used? 4 MB can be covered by a single
> > descriptor if the data is physically contiguous in memory, so this data
> > doesn't demonstrate a need for more descriptors.
> 
> No, in the last couple years there was apparently no kernel version that used
> just one descriptor, nor did my benchmarked version. Even though the Linux 9p
> client uses (yet) simple linear buffers (contiguous physical memory) on 9p
> client level, these are however split into PAGE_SIZE chunks by function
> pack_sg_list() [1] before being fed to virtio level:
> 
> static unsigned int rest_of_page(void *data)
> {
>       return PAGE_SIZE - offset_in_page(data);
> }
> ...
> static int pack_sg_list(struct scatterlist *sg, int start,
>                       int limit, char *data, int count)
> {
>       int s;
>       int index = start;
> 
>       while (count) {
>               s = rest_of_page(data);
>               ...
>               sg_set_buf(&sg[index++], data, s);
>               count -= s;
>               data += s;
>       }
>       ...
> }
> 
> [1] 
> https://github.com/torvalds/linux/blob/19901165d90fdca1e57c9baa0d5b4c63d15c476a/net/9p/trans_virtio.c#L171
> 
> So when sending 4MB over virtio wire, it would yield in 1k descriptors ATM.
> 
> I have wondered about this before, but did not question it, because due to the
> cross-platform nature I couldn't say for certain whether that's probably
> needed somewhere. I mean for the case virtio-PCI I know for sure that one
> descriptor (i.e. >PAGE_SIZE) would be fine, but I don't know if that applies
> to all buses and architectures.

VIRTIO does not limit descriptor the descriptor len field to PAGE_SIZE,
so I don't think there is a limit at the VIRTIO level.

If this function coalesces adjacent pages then the descriptor chain
length issues could be reduced.

> > > But again, this is not just about performance. My conclusion as described
> > > in my previous email is that virtio currently squeezes
> > > 
> > >   "max. simultanious amount of bulk messages"
> > > 
> > > vs.
> > > 
> > >   "max. bulk data transmission size per bulk messaage"
> > > 
> > > into the same configuration parameter, which is IMO inappropriate and
> > > hence
> > > splitting them into 2 separate parameters when creating a queue makes
> > > sense, independent of the performance benchmarks.
> > > 
> > > [1]
> > > https://lore.kernel.org/netdev/cover.1632327421.git.linux_oss@crudebyte.c
> > > om/
> > Some devices effectively already have this because the device advertises
> > a maximum number of descriptors via device-specific mechanisms like the
> > struct virtio_blk_config seg_max field. But today these fields can only
> > reduce the maximum descriptor chain length because the spec still limits
> > the length to Queue Size.
> > 
> > We can build on this approach to raise the length above Queue Size. This
> > approach has the advantage that the maximum number of segments isn't per
> > device or per virtqueue, it's fine-grained. If the device supports two
> > requests types then different max descriptor chain limits could be given
> > for them by introducing two separate configuration space fields.
> > 
> > Here are the corresponding spec changes:
> > 
> > 1. A new feature bit called VIRTIO_RING_F_LARGE_INDIRECT_DESC is added
> >    to indicate that indirect descriptor table size and maximum
> >    descriptor chain length are not limited by Queue Size value. (Maybe
> >    there still needs to be a limit like 2^15?)
> 
> Sounds good to me!
> 
> AFAIK it is effectively limited to 2^16 because of vring_desc->next:
> 
> /* Virtio ring descriptors: 16 bytes.  These can chain together via "next". */
> struct vring_desc {
>         /* Address (guest-physical). */
>         __virtio64 addr;
>         /* Length. */
>         __virtio32 len;
>         /* The flags as indicated above. */
>         __virtio16 flags;
>         /* We chain unused descriptors via this, too */
>         __virtio16 next;
> };

Yes, Split Virtqueues have a fundamental limit on indirect table size
due to the "next" field. Packed Virtqueue descriptors don't have a
"next" field so descriptor chains could be longer in theory (currently
forbidden by the spec).

> > One thing that's messy is that we've been discussing the maximum
> > descriptor chain length but 9p has the "msize" concept, which isn't
> > aware of contiguous memory. It may be necessary to extend the 9p driver
> > code to size requests not just according to their length in bytes but
> > also according to the descriptor chain length. That's how the Linux
> > block layer deals with queue limits (struct queue_limits max_segments vs
> > max_hw_sectors).
> 
> Hmm, can't follow on that one. For what should that be needed in case of 9p?
> My plan was to limit msize by 9p client simply at session start to whatever is
> the max. amount virtio descriptors supported by host and using PAGE_SIZE as
> size per descriptor, because that's what 9p client actually does ATM (see
> above). So you think that should be changed to e.g. just one descriptor for
> 4MB, right?

Limiting msize to the 9p transport device's maximum number of
descriptors is conservative (i.e. 128 descriptors = 512 KB msize)
because it doesn't take advantage of contiguous memory. I suggest
leaving msize alone, adding a separate limit at which requests are split
according to the maximum descriptor chain length, and tweaking
pack_sg_list() to coalesce adjacent pages.

That way msize can be large without necessarily using lots of
descriptors (depending on the memory layout).

Stefan

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