Re: [Qemu-devel] [PATCH v6 03/12] dataplane: add host memory mapping code

[Michael S. Tsirkin]

On Fri, Dec 14, 2012 at 12:45:16PM +0100, Stefan Hajnoczi wrote:
> On Wed, Dec 12, 2012 at 4:49 PM, Michael S. Tsirkin <address@hidden> wrote:
> > On Wed, Dec 12, 2012 at 04:34:21PM +0100, Stefan Hajnoczi wrote:
> >> On Tue, Dec 11, 2012 at 08:09:56PM +0200, Michael S. Tsirkin wrote:
> >> > On Tue, Dec 11, 2012 at 10:32:28AM -0600, Anthony Liguori wrote:
> >> > > "Michael S. Tsirkin" <address@hidden> writes:
> >> > >
> >> > > > On Tue, Dec 11, 2012 at 04:27:49PM +0100, Stefan Hajnoczi wrote:
> >> > > >> On Tue, Dec 11, 2012 at 3:13 PM, Michael S. Tsirkin 
> >> > > >> <address@hidden> wrote:
> >> > > >> > On Mon, Dec 10, 2012 at 02:09:36PM +0100, Stefan Hajnoczi wrote:
> >> > > >> >> The data plane thread needs to map guest physical addresses to 
> >> > > >> >> host
> >> > > >> >> pointers.  Normally this is done with cpu_physical_memory_map() 
> >> > > >> >> but the
> >> > > >> >> function assumes the global mutex is held.  The data plane 
> >> > > >> >> thread does
> >> > > >> >> not touch the global mutex and therefore needs a thread-safe 
> >> > > >> >> memory
> >> > > >> >> mapping mechanism.
> >> > > >> >>
> >> > > >> >> Hostmem registers a MemoryListener similar to how vhost collects 
> >> > > >> >> and
> >> > > >> >> pushes memory region information into the kernel.  There is a
> >> > > >> >> fine-grained lock on the regions list which is held during 
> >> > > >> >> lookup and
> >> > > >> >> when installing a new regions list.
> >> > > >> >
> >> > > >> > Can we export and reuse the vhost code for this?
> >> > > >> > I think you will find this advantageous when you add migration
> >> > > >> > support down the line.
> >> > > >> > And if you find it necessary to use MemoryListener e.g. for 
> >> > > >> > performance
> >> > > >> > reasons, then vhost will likely benefit too.
> >> > > >>
> >> > > >> It's technically possible and not hard to do but it prevents
> >> > > >> integrating deeper with core QEMU as the memory API becomes
> >> > > >> thread-safe.
> >> > > >>
> >> > > >> There are two ways to implement dirty logging:
> >> > > >> 1. The vhost log approach which syncs dirty information 
> >> > > >> periodically.
> >> > > >> 2. A cheap thread-safe way to mark dirty outside the global mutex,
> >> > > >> i.e. a thread-safe memory_region_set_dirty().
> >> > > >
> >> > > > You don't normally want to dirty the whole region,
> >> > > > you want to do this to individual pages.
> >> > > >
> >> > > >> If we can get thread-safe guest memory load/store in QEMU then #2 is
> >> > > >> included.  We can switch to using hw/virtio.c instead of
> >> > > >> hw/dataplane/vring.c, we get dirty logging for free, we can drop
> >> > > >> hostmem.c completely, etc.
> >> > > >>
> >> > > >> Stefan
> >> > > >
> >> > > > So why not reuse existing code? If you drop it later it won't
> >> > > > matter what you used ...
> >> > >
> >> > > Let's not lose sight of the forest for the trees here...
> >> > >
> >> > > This whole series is not reusing existing code.  That's really the 
> >> > > whole
> >> > > point.
> >> > >
> >> > > The point is to take the code (duplication and all) and then do all of
> >> > > the refactoring to use common code in the tree itself.
> >> > >
> >> > > If we want to put this in a hw/staging/ directory, that's fine by me
> >> > > too.
> >> > >
> >> > > Regards,
> >> > >
> >> > > Anthony Liguori
> >> >
> >> > Yes I agree. I think lack of handling for cross regin descriptors
> >> > bothers me a bit more.
> >>
> >> The two things you've mentioned both aren't handled by hw/virtio.c:
> >>
> >> 1. Issue: Indirect descriptors have no alignment restrictions and can
> >>    cross regions.
> >>
> >>    hw/virtio.c uses vring_desc_flags() and other accessor functions,
> >>    which do lduw_phys() - there is no memory region boundary checking
> >>    here.
> >
> > Since addresses are aligned this one is fine I think.
> >
> >> 2. Issue: Virtio buffers can cross memory region boundaries.
> >>
> >>    hw/virtio.c maps buffers 1:1 using virtqueue_map_sg() and exits if
> >>    mapping fails.  It does not split buffers if they cross a memory
> >>    region.
> >>
> >> These are definitely ugly corner cases but hw/virtio.c is proof that
> >> we're not hitting them in practice.
> >>
> >> Stefan
> >
> > Yes, this one seems ugly. Maybe add a TODO?
> >
> > OK let's assume we want to put it in staging/
> > I worry about the virtio-blk changes being isolated.
> > Can you put ifdef CONFIG_VIRTIO_BLK_DATA_PLANE around
> > them all to avoid dependency on that header completely
> > if configured out?
> 
> Okay, I'll move the #ifdefs.  I like the stubs in the header file
> because it reduces the amount of #ifdefs, but this is easy to change.
> 
> Stefan

Okay.
Another option (if you prefer stubs) is to add a stub for access to
s->dataplane field, and surround just the field with ifdefs.
As it is, this code:
        if (s->dataplane) {
                return;
        }
can't be compiled out since compiler is not smart enough to
figure out dataplane is never set.
-- 
MST