Re: [PATCH v2 2/2] virtio: fix vq # for balloon

[Daniel Verkamp]

On Wed, Jul 10, 2024 at 1:39 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> On Wed, Jul 10, 2024 at 12:58:11PM -0700, Daniel Verkamp wrote:
> > On Wed, Jul 10, 2024 at 11:39 AM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> > >
> > > On Wed, Jul 10, 2024 at 11:12:34AM -0700, Daniel Verkamp wrote:
> > > > On Wed, Jul 10, 2024 at 4:43 AM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> > > > >
> > > > > virtio balloon communicates to the core that in some
> > > > > configurations vq #s are non-contiguous by setting name
> > > > > pointer to NULL.
> > > > >
> > > > > Unfortunately, core then turned around and just made them
> > > > > contiguous again. Result is that driver is out of spec.
> > > >
> > > > Thanks for fixing this - I think the overall approach of the patch looks good.
> > > >
> > > > > Implement what the API was supposed to do
> > > > > in the 1st place. Compatibility with buggy hypervisors
> > > > > is handled inside virtio-balloon, which is the only driver
> > > > > making use of this facility, so far.
> > > >
> > > > In addition to virtio-balloon, I believe the same problem also affects
> > > > the virtio-fs device, since queue 1 is only supposed to be present if
> > > > VIRTIO_FS_F_NOTIFICATION is negotiated, and the request queues are
> > > > meant to be queue indexes 2 and up. From a look at the Linux driver
> > > > (virtio_fs.c), it appears like it never acks VIRTIO_FS_F_NOTIFICATION
> > > > and assumes that request queues start at index 1 rather than 2, which
> > > > looks out of spec to me, but the current device implementations (that
> > > > I am aware of, anyway) are also broken in the same way, so it ends up
> > > > working today. Queue numbering in a spec-compliant device and the
> > > > current Linux driver would mismatch; what the driver considers to be
> > > > the first request queue (index 1) would be ignored by the device since
> > > > queue index 1 has no function if F_NOTIFICATION isn't negotiated.
> > >
> > >
> > > Oh, thanks a lot for pointing this out!
> > >
> > > I see so this patch is no good as is, we need to add a workaround for
> > > virtio-fs first.
> > >
> > > QEMU workaround is simple - just add an extra queue. But I did not
> > > reasearch how this would interact with vhost-user.
> > >
> > > From driver POV, I guess we could just ignore queue # 1 - would that be
> > > ok or does it have performance implications?
> >
> > As a driver workaround for non-compliant devices, I think ignoring the
> > first request queue would be a reasonable approach if the device's
> > config advertises num_request_queues > 1. Unfortunately, both
> > virtiofsd and crosvm's virtio-fs device have hard-coded
> > num_request_queues =1, so this won't help with those existing devices.
>
> Do they care what the vq # is though?
> We could do some magic to translate VQ #s in qemu.
>
>
> > Maybe there are other devices that we would need to consider as well;
> > commit 529395d2ae64 ("virtio-fs: add multi-queue support") quotes
> > benchmarks that seem to be from a different virtio-fs implementation
> > that does support multiple request queues, so the workaround could
> > possibly be used there.
> >
> > > Or do what I did for balloon here: try with spec compliant #s first,
> > > if that fails then assume it's the spec issue and shift by 1.
> >
> > If there is a way to "guess and check" without breaking spec-compliant
> > devices, that sounds reasonable too; however, I'm not sure how this
> > would work out in practice: an existing non-compliant device may fail
> > to start if the driver tries to enable queue index 2 when it only
> > supports one request queue,
>
> You don't try to enable queue - driver starts by checking queue size.
> The way my patch works is that it assumes a non existing queue has
> size 0 if not available.
>
> This was actually a documented way to check for PCI and MMIO:
>         Read the virtqueue size from queue_size. This controls how big the virtqueue is (see 2.6 Virtqueues).
>         If this field is 0, the virtqueue does not exist.
> MMIO:
>         If the returned value is zero (0x0) the queue is not available.
>
> unfortunately not for CCW, but I guess CCW implementations outside
> of QEMU are uncommon enough that we can assume it's the same?
>
>
> To me the above is also a big hint that drivers are allowed to
> query size for queues that do not exist.

Ah, that makes total sense - detecting queue presence by non-zero
queue size sounds good to me, and it should work in the normal virtio
device case.

I am not sure about vhost-user, since there is no way for the
front-end to ask the back-end for a queue's size; the confusingly
named VHOST_USER_SET_VRING_NUM allows the front-end to configure the
size of a queue, but there's no corresponding GET message.

> > and a spec-compliant device would probably
> > balk if the driver tries to enable queue 1 but does not negotiate
> > VIRTIO_FS_F_NOTIFICATION. If there's a way to reset and retry the
> > whole virtio device initialization process if a device fails like
> > this, then maybe it's feasible. (Or can the driver tweak the virtqueue
> > configuration and try to set DRIVER_OK repeatedly until it works? It's
> > not clear to me if this is allowed by the spec, or what device
> > implementations actually do in practice in this scenario.)
> >
> > Thanks,
> > -- Daniel
>
> My patch starts with a spec compliant behaviour. If that fails,
> try non-compliant one as a fallback.

Got it, that sounds reasonable to me given the explanation above.

Thanks,
-- Daniel