Re: [PATCH 7/7] vhost-vdpa: backend feature should set only once

[Si-Wei Liu]

On 3/31/2022 1:02 AM, Eugenio Perez Martin wrote:
> On Thu, Mar 31, 2022 at 1:03 AM Si-Wei Liu <si-wei.liu@oracle.com> wrote:
> > On 3/30/2022 12:01 PM, Eugenio Perez Martin wrote:
> > > On Wed, Mar 30, 2022 at 8:33 AM Si-Wei Liu <si-wei.liu@oracle.com> wrote:
> > > > The vhost_vdpa_one_time_request() branch in
> > > > vhost_vdpa_set_backend_cap() incorrectly sends down
> > > > iotls on vhost_dev with non-zero index. This may
> > > > end up with multiple VHOST_SET_BACKEND_FEATURES
> > > > ioctl calls sent down on the vhost-vdpa fd that is
> > > > shared between all these vhost_dev's.
> > > Not only that. This means that qemu thinks the device supports iotlb
> > > batching as long as the device does not have cvq. If vdpa does not
> > > support batching, it will return an error later with no possibility of
> > > doing it ok.
> > I think the implicit assumption here is that the caller should back off
> > to where it was if it comes to error i.e. once the first
> > vhost_dev_set_features call gets an error, vhost_dev_start() will fail
> > straight.
> Sorry, I don't follow you here, and maybe my message was not clear enough.
>
> What I meant is that your patch fixes another problem not stated in
> the message: it is not possible to initialize a net vdpa device that
> does not have cvq and does not support iotlb batches without it. Qemu
> will assume that the device supports batching, so the write of
> VHOST_IOTLB_BATCH_BEGIN will fail.
This is not what I see from the code? For e.g. 
vhost_vdpa_iotlb_batch_begin_once() has the following:

 140     if (v->dev->backend_cap & (0x1ULL << 
VHOST_BACKEND_F_IOTLB_BATCH) &&
 141         !v->iotlb_batch_begin_sent) {
 142         vhost_vdpa_listener_begin_batch(v);
 143     }

If backend_cap doesn't contain the VHOST_BACKEND_F_IOTLB_BATCH bit, QEMU 
shouldn't send down VHOST_IOTLB_BATCH_BEGIN...

Noted in vhost_vdpa_set_backend_cap(), VHOST_GET_BACKEND_FEATURES was 
supposed to get the backend capability from the kernel ahead of the 
VHOST_SET_BACKEND_FEATURES call. In which case of your concern, at least 
feature VHOST_BACKEND_F_IOTLB_MSG_V2 should be successfully returned and 
stored in the backend_cap, even if the VHOST_SET_BACKEND_FEATURES ioctl 
was missed in between. Hence the resulting backend_cap shouldn't have 
the VHOST_BACKEND_F_IOTLB_BATCH bit set. What am I missing here?


>   I didn't test what happens next but
> it probably cannot continue.
>
> In that regard, this commit needs to be marked as "Fixes: ...", either
> ("a5bd058 vhost-vdpa: batch updating IOTLB mappings") or maybe better
> ("4d191cf vhost-vdpa: classify one time request"). We have a
> regression if we introduce both, or the second one and the support of
> any other backend feature.
Sure, it's not that I am unwilling to add the "Fixes" tag, though I'd 
like to make sure if the worry is real upfront. Thanks for pointing it 
out anyway.

Thanks,
-Siwei

> > Noted that the VHOST_SET_BACKEND_FEATURES ioctl is not per-vq
> > and it doesn't even need to. There seems to me no possibility for it to
> > fail in a way as thought here. The capture is that IOTLB batching is at
> > least a vdpa device level backend feature, if not per-kernel. Same as
> > IOTLB_MSG_V2.
> At this moment it is per-kernel, yes. With your patch there is no need
> to fail because of the lack of _F_IOTLB_BATCH, the code should handle
> this case ok.
>
> But if VHOST_GET_BACKEND_FEATURES returns no support for
> VHOST_BACKEND_F_IOTLB_MSG_V2, the qemu code will happily send v2
> messages anyway. This has nothing to do with the patch, I'm just
> noting it here.
>
> In that case, maybe it is better to return something like -ENOTSUP?
>
> Thanks!
>
> > -Siwei
> >
> > >    Some open questions:
> > >
> > > Should we make the vdpa driver return error as long as a feature is
> > > used but not set by qemu, or let it as undefined? I guess we have to
> > > keep the batching at least without checking so the kernel supports old
> > > versions of qemu.
> > >
> > > On the other hand, should we return an error if IOTLB_MSG_V2 is not
> > > supported here? We're basically assuming it in other functions.
> > >
> > > > To fix it, send down ioctl only once via the first
> > > > vhost_dev with index 0. Toggle the polarity of the
> > > > vhost_vdpa_one_time_request() test would do the trick.
> > > >
> > > > Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
> > > Acked-by: Eugenio Pérez <eperezma@redhat.com>
> > >
> > > > ---
> > > >    hw/virtio/vhost-vdpa.c | 2 +-
> > > >    1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/hw/virtio/vhost-vdpa.c b/hw/virtio/vhost-vdpa.c
> > > > index c5ed7a3..27ea706 100644
> > > > --- a/hw/virtio/vhost-vdpa.c
> > > > +++ b/hw/virtio/vhost-vdpa.c
> > > > @@ -665,7 +665,7 @@ static int vhost_vdpa_set_backend_cap(struct vhost_dev *dev)
> > > >
> > > >        features &= f;
> > > >
> > > > -    if (vhost_vdpa_one_time_request(dev)) {
> > > > +    if (!vhost_vdpa_one_time_request(dev)) {
> > > >            r = vhost_vdpa_call(dev, VHOST_SET_BACKEND_FEATURES, &features);
> > > >            if (r) {
> > > >                return -EFAULT;
> > > > --
> > > > 1.8.3.1