On Fri, Apr 1, 2022 at 12:18 PM Si-Wei Liu <si-wei....@oracle.com> wrote:
>
>
>
> On 3/31/2022 7:39 PM, Jason Wang wrote:
> > On Thu, Mar 31, 2022 at 5:20 PM Eugenio Perez Martin
> > <epere...@redhat.com> wrote:
> >> On Thu, Mar 31, 2022 at 10:54 AM Jason Wang <jasow...@redhat.com> wrote:
> >>>
> >>> 在 2022/3/31 下午4:02, Eugenio Perez Martin 写道:
> >>>> On Thu, Mar 31, 2022 at 1:03 AM Si-Wei Liu <si-wei....@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....@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. I didn't test what happens next but
> >>>> it probably cannot continue.
> >>>
> >>> So you mean we actually didn't call VHOST_SET_BACKEND_CAP in this case.
> >>> Fortunately, kernel didn't check the backend cap when accepting batching
> >>> hints.
> >>>
> >>> We are probably fine?
> >>>
> >> We're fine as long as the vdpa driver in the kernel effectively
> >> supports batching. If not, qemu will try to batch, and it will fail.
> >>
> >> It was introduced in v5.9, so qemu has not supported kernel <5.9 since
> >> we introduced multiqueue support (I didn't test). Unless we apply this
> >> patch. That's the reason it should be marked as fixed and backported
> >> to stable IMO.
> > Ok, so it looks to me we have more issues.
> >
> > In vhost_vdpa_set_backend_cap() we fail when
> > VHOST_VDPA_GET_BACKEND_FEATURES fails. This breaks the older kernel
> > since that ioctl is introduced in
> >
> > 653055b9acd4 ("vhost-vdpa: support get/set backend features")
> Yep, the GET/SET_BACKEND ioctl pair got introduced together in this
> exact commit.
> >
> > We should:
> >
> > 1) make it work by not failing the vhost_vdpa_set_backend_cap() and
> > assuming MSG_V2.
> This issue is orthogonal with my fix, which was pre-existing before the
> multiqueue support. I believe there should be another separate patch to
> fix QEMU for pre-GET/SET_BACKEND kernel.
Right.
>
> > 2) check the batching support in vhost_vdpa_listener_begin_batch()
> > instead of trying to set VHOST_IOTLB_BATCH_BEGIN uncondtionally
> This is non-issue since VHOST_BACKEND_F_IOTLB_BATCH is already validated
> in the caller vhost_vdpa_iotlb_batch_begin_once().
Yes, I miss that optimization.
Thanks
>
> -Siwei
> >
> > Thanks
> >
> >> Thanks!
> >>
> >>> Thanks
> >>>
> >>>
> >>>> 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.
> >>>>
> >>>>> 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....@oracle.com>
> >>>>>> Acked-by: Eugenio Pérez <epere...@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
> >>>>>>>
>
