Re: [PATCH 17/31] vdpa: adapt vhost_ops callbacks to svq

[Eugenio Perez Martin]

On Mon, Feb 21, 2022 at 8:15 AM Jason Wang <jasowang@redhat.com> wrote:
>
>
> 在 2022/2/18 上午1:13, Eugenio Perez Martin 写道:
> > On Tue, Feb 8, 2022 at 4:58 AM Jason Wang <jasowang@redhat.com> wrote:
> >>
> >> 在 2022/2/1 上午2:58, Eugenio Perez Martin 写道:
> >>> On Sun, Jan 30, 2022 at 5:03 AM Jason Wang <jasowang@redhat.com> wrote:
> >>>> 在 2022/1/22 上午4:27, Eugenio Pérez 写道:
> >>>>> First half of the buffers forwarding part, preparing vhost-vdpa
> >>>>> callbacks to SVQ to offer it. QEMU cannot enable it at this moment, so
> >>>>> this is effectively dead code at the moment, but it helps to reduce
> >>>>> patch size.
> >>>>>
> >>>>> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> >>>>> ---
> >>>>>     hw/virtio/vhost-shadow-virtqueue.h |   2 +-
> >>>>>     hw/virtio/vhost-shadow-virtqueue.c |  21 ++++-
> >>>>>     hw/virtio/vhost-vdpa.c             | 133 
> >>>>> ++++++++++++++++++++++++++---
> >>>>>     3 files changed, 143 insertions(+), 13 deletions(-)
> >>>>>
> >>>>> diff --git a/hw/virtio/vhost-shadow-virtqueue.h 
> >>>>> b/hw/virtio/vhost-shadow-virtqueue.h
> >>>>> index 035207a469..39aef5ffdf 100644
> >>>>> --- a/hw/virtio/vhost-shadow-virtqueue.h
> >>>>> +++ b/hw/virtio/vhost-shadow-virtqueue.h
> >>>>> @@ -35,7 +35,7 @@ size_t vhost_svq_device_area_size(const 
> >>>>> VhostShadowVirtqueue *svq);
> >>>>>
> >>>>>     void vhost_svq_stop(VhostShadowVirtqueue *svq);
> >>>>>
> >>>>> -VhostShadowVirtqueue *vhost_svq_new(void);
> >>>>> +VhostShadowVirtqueue *vhost_svq_new(uint16_t qsize);
> >>>>>
> >>>>>     void vhost_svq_free(VhostShadowVirtqueue *vq);
> >>>>>
> >>>>> diff --git a/hw/virtio/vhost-shadow-virtqueue.c 
> >>>>> b/hw/virtio/vhost-shadow-virtqueue.c
> >>>>> index f129ec8395..7c168075d7 100644
> >>>>> --- a/hw/virtio/vhost-shadow-virtqueue.c
> >>>>> +++ b/hw/virtio/vhost-shadow-virtqueue.c
> >>>>> @@ -277,9 +277,17 @@ void vhost_svq_stop(VhostShadowVirtqueue *svq)
> >>>>>     /**
> >>>>>      * Creates vhost shadow virtqueue, and instruct vhost device to use 
> >>>>> the shadow
> >>>>>      * methods and file descriptors.
> >>>>> + *
> >>>>> + * @qsize Shadow VirtQueue size
> >>>>> + *
> >>>>> + * Returns the new virtqueue or NULL.
> >>>>> + *
> >>>>> + * In case of error, reason is reported through error_report.
> >>>>>      */
> >>>>> -VhostShadowVirtqueue *vhost_svq_new(void)
> >>>>> +VhostShadowVirtqueue *vhost_svq_new(uint16_t qsize)
> >>>>>     {
> >>>>> +    size_t desc_size = sizeof(vring_desc_t) * qsize;
> >>>>> +    size_t device_size, driver_size;
> >>>>>         g_autofree VhostShadowVirtqueue *svq = 
> >>>>> g_new0(VhostShadowVirtqueue, 1);
> >>>>>         int r;
> >>>>>
> >>>>> @@ -300,6 +308,15 @@ VhostShadowVirtqueue *vhost_svq_new(void)
> >>>>>         /* Placeholder descriptor, it should be deleted at set_kick_fd 
> >>>>> */
> >>>>>         event_notifier_init_fd(&svq->svq_kick, INVALID_SVQ_KICK_FD);
> >>>>>
> >>>>> +    svq->vring.num = qsize;
> >>>> I wonder if this is the best. E.g some hardware can support up to 32K
> >>>> queue size. So this will probably end up with:
> >>>>
> >>>> 1) SVQ use 32K queue size
> >>>> 2) hardware queue uses 256
> >>>>
> >>> In that case SVQ vring queue size will be 32K and guest's vring can
> >>> negotiate any number with SVQ equal or less than 32K,
> >>
> >> Sorry for being unclear what I meant is actually
> >>
> >> 1) SVQ uses 32K queue size
> >>
> >> 2) guest vq uses 256
> >>
> >> This looks like a burden that needs extra logic and may damage the
> >> performance.
> >>
> > Still not getting this point.
> >
> > An available guest buffer, although contiguous in GPA/GVA, can expand
> > in multiple buffers if it's not contiguous in qemu's VA (by the while
> > loop in virtqueue_map_desc [1]). In that scenario it is better to have
> > "plenty" of SVQ buffers.
>
>
> Yes, but this case should be rare. So in this case we should deal with
> overrun on SVQ, that is
>
> 1) SVQ is full
> 2) guest VQ isn't
>
> We need to
>
> 1) check the available buffer slots
> 2) disable guest kick and wait for the used buffers
>
> But it looks to me the current code is not ready for dealing with this case?
>

Yes it deals, that's the meaning of svq->next_guest_avail_elem.

>
> >
> > I'm ok if we decide to put an upper limit though, or if we decide not
> > to handle this situation. But we would leave out valid virtio drivers.
> > Maybe to set a fixed upper limit (1024?)? To add another parameter
> > (x-svq-size-n=N)?
> >
> > If you mean we lose performance because memory gets more sparse I
> > think the only possibility is to limit that way.
>
>
> If guest is not using 32K, having a 32K for svq may gives extra stress
> on the cache since we will end up with a pretty large working set.
>

That might be true. My guess is that it should not matter, since SVQ
and the guest's vring will have the same numbers of scattered buffers
and the avail / used / packed ring will be consumed more or less
sequentially. But I haven't tested.

I think it's better to add an upper limit (either fixed or in the
qemu's backend's cmdline) later if we see that this is a problem.
Another solution now would be to get the number from the frontend
device cmdline instead of from the vdpa device. I'm ok with that, but
it doesn't delete the svq->next_guest_avail_elem processing, and it
comes with disadvantages in my opinion. More below.

>
> >
> >> And this can lead other interesting situation:
> >>
> >> 1) SVQ uses 256
> >>
> >> 2) guest vq uses 1024
> >>
> >> Where a lot of more SVQ logic is needed.
> >>
> > If we agree that a guest descriptor can expand in multiple SVQ
> > descriptors, this should be already handled by the previous logic too.
> >
> > But this should only happen in case that qemu is launched with a "bad"
> > cmdline, isn't it?
>
>
> This seems can happen when we use -device
> virtio-net-pci,tx_queue_size=1024 with a 256 size vp_vdpa device at least?
>

I'm going to use the rx queue here since it's more accurate, tx has
its own limit separately.

If we use rx_queue_size=256 in L0 and rx_queue_size=1024 in L1 with no
SVQ, L0 qemu will happily accept 1024 as size when L1 qemu writes that
value at vhost_virtqueue_start. I'm not sure what would happen with a
real device, my guess is that the device will fail somehow. That's
what I meant with a "bad cmdline", I should have been more specific.

If we add SVQ to the mix, the guest first negotiates the 1024 with the
qemu device model. After that, vhost.c will try to write 1024 too but
this is totally ignored by this patch's changes at
vhost_vdpa_set_vring_num. Finally, SVQ will set 256 as a ring size to
the device, since it's the read value from the device, leading to your
scenario. So SVQ effectively isolates both sides and makes possible
the communication, even with a device that does not support so many
descriptors.

But SVQ already handles this case: It's the same as if the buffers are
fragmented in HVA and queue size is equal at both sides. That's why I
think SVQ size should depend on the backend device's size, not
frontend cmdline.

Thanks!

>
> >
> > If I run that example with vp_vdpa, L0 qemu will happily accept 1024
> > as a queue size [2]. But if the vdpa device maximum queue size is
> > effectively 256, this will result in an error: We're not exposing it
> > to the guest at any moment but with qemu's cmdline.
> >
> >>> including 256.
> >>> Is that what you mean?
> >>
> >> I mean, it looks to me the logic will be much more simplified if we just
> >> allocate the shadow virtqueue with the size what guest can see (guest
> >> vring).
> >>
> >> Then we don't need to think if the difference of the queue size can have
> >> any side effects.
> >>
> > I think that we cannot avoid that extra logic unless we force GPA to
> > be contiguous in IOVA. If we are sure the guest's buffers cannot be at
> > more than one descriptor in SVQ, then yes, we can simplify things. If
> > not, I think we are forced to carry all of it.
>
>
> Yes, I agree, the code should be robust to handle any case.
>
> Thanks
>
>
> >
> > But if we prove it I'm not opposed to simplifying things and making
> > head at SVQ == head at guest.
> >
> > Thanks!
> >
> > [1] 
> > https://gitlab.com/qemu-project/qemu/-/blob/17e31340/hw/virtio/virtio.c#L1297
> > [2] But that's not the whole story: I've been running limited in tx
> > descriptors because of virtio_net_max_tx_queue_size, which predates
> > vdpa. I'll send a patch to also un-limit it.
> >
> >>> If with hardware queues you mean guest's vring, not sure why it is
> >>> "probably 256". I'd say that in that case with the virtio-net kernel
> >>> driver the ring size will be the same as the device export, for
> >>> example, isn't it?
> >>>
> >>> The implementation should support any combination of sizes, but the
> >>> ring size exposed to the guest is never bigger than hardware one.
> >>>
> >>>> ? Or we SVQ can stick to 256 but this will this cause trouble if we want
> >>>> to add event index support?
> >>>>
> >>> I think we should not have any problem with event idx. If you mean
> >>> that the guest could mark more buffers available than SVQ vring's
> >>> size, that should not happen because there must be less entries in the
> >>> guest than SVQ.
> >>>
> >>> But if I understood you correctly, a similar situation could happen if
> >>> a guest's contiguous buffer is scattered across many qemu's VA chunks.
> >>> Even if that would happen, the situation should be ok too: SVQ knows
> >>> the guest's avail idx and, if SVQ is full, it will continue forwarding
> >>> avail buffers when the device uses more buffers.
> >>>
> >>> Does that make sense to you?
> >>
> >> Yes.
> >>
> >> Thanks
> >>
>