Re: [PATCH V2] vhost_vdpa: no need to fetch vring base when poweroff

["Zhu, Lingshan" <lingshan.zhu@intel.com>]

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On 7/11/2023 3:34 PM, Jason Wang wrote:
>
>
> On Tue, Jul 11, 2023 at 3:25 PM Eugenio Perez Martin 
> <eperezma@redhat.com> wrote:
>
>     On Tue, Jul 11, 2023 at 9:05 AM Jason Wang <jasowang@redhat.com>
>     wrote:
>     >
>     > On Tue, Jul 11, 2023 at 12:09 PM Zhu, Lingshan
>     <lingshan.zhu@intel.com> wrote:
>     > >
>     > >
>     > >
>     > > On 7/11/2023 10:50 AM, Jason Wang wrote:
>     > > > On Mon, Jul 10, 2023 at 4:53 PM Zhu Lingshan
>     <lingshan.zhu@intel.com> wrote:
>     > > >> In the poweroff routine, no need to fetch last available index.
>     > > >>
>     > > > This is because there's no concept of shutdown in the vhost
>     layer, it
>     > > > only knows start and stop.
>     > > >
>     > > >> This commit also provides a better debug message in the vhost
>     > > >> caller vhost_virtqueue_stop,
>     > > > A separate patch is better.
>     > > OK
>     > > >
>     > > >> because if vhost does not fetch
>     > > >> the last avail idx successfully, maybe the device does not
>     > > >> suspend, vhost will sync last avail idx to vring used idx as a
>     > > >> work around, not a failure.
>     > > > This only happens if we return a negative value?
>     > > Yes
>     > > >
>     > > >> Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
>     > > >> ---
>     > > >>   hw/virtio/vhost-vdpa.c | 10 ++++++++++
>     > > >>   hw/virtio/vhost.c      |  2 +-
>     > > >>   2 files changed, 11 insertions(+), 1 deletion(-)
>     > > >>
>     > > >> diff --git a/hw/virtio/vhost-vdpa.c b/hw/virtio/vhost-vdpa.c
>     > > >> index 3c575a9a6e..10b445f64e 100644
>     > > >> --- a/hw/virtio/vhost-vdpa.c
>     > > >> +++ b/hw/virtio/vhost-vdpa.c
>     > > >> @@ -26,6 +26,7 @@
>     > > >>   #include "cpu.h"
>     > > >>   #include "trace.h"
>     > > >>   #include "qapi/error.h"
>     > > >> +#include "sysemu/runstate.h"
>     > > >>
>     > > >>   /*
>     > > >>    * Return one past the end of the end of section. Be
>     careful with uint64_t
>     > > >> @@ -1391,6 +1392,15 @@ static int
>     vhost_vdpa_get_vring_base(struct vhost_dev *dev,
>     > > >>       struct vhost_vdpa *v = dev->opaque;
>     > > >>       int ret;
>     > > >>
>     > > >> +    if (runstate_check(RUN_STATE_SHUTDOWN)) {
>     > > >> +        /*
>     > > >> +         * Some devices do not support this call properly,
>     > > >> +         * and we don't need to retrieve the indexes
>     > > >> +         * if it is shutting down
>     > > >> +         */
>     > > >> +        return 0;
>     > > > Checking runstate in the vhost code seems like a layer
>     violation.
>     > > >
>     > > > What happens without this patch?
>     > > vhost tries to fetch vring base,
>     > > vhost_vdpa needs suspend the device before retrieving
>     last_avail_idx.
>     > > However not all devices can support .suspend properly so this call
>     > > may fail.
>     >
>     > I think this is where I'm lost. If the device doesn't support
>     > suspending, any reason we only try to fix the case of shutdown?
>     >
>     > Btw, the fail is intended:
>     >
>     >     if (!v->suspended) {
>     >         /*
>     >          * Cannot trust in value returned by device, let vhost
>     recover used
>     >          * idx from guest.
>     >          */
>     >         return -1;
>     >     }
>     >
>
>     The fail is intended, but to recover the last used idx, either from
>     device or from the guest, is only useful in the case of migration.
>
>
> Note that we had userspace devices like VDUSE now, so it could be 
> useful in the case of a VDUSE daemon crash or reconnect.
This code blcok is for vhost_vdpa backend, and I think vduse is another 
code path.
Return a guest used idx may be a good idea but as Eugenio pointed out 
that may duplicate the code.
>
>
>     I think the main problem is the error message, actually. But I think
>     it has no use either to recover last_avail_idx or print a debug
>     message if we're not migrating. Another solution would be to recover
>     it from the guest at vhost_vdpa_get_vring_base, but I don't like the
>     duplication.
>
>     > And if we return to success here, will we go to set an uninitialized
>     > last avail idx?
>     >
>
>     It will be either the default value (is set to 0 at
>     __virtio_queue_reset) or the one received from a migration (at
>     virtio_load).
>
>
> 0 is even sub-optimal than the index used. Anyhow, VHOST_DEBUG should 
> not be enabled for production environments.
Returning 0 sounds like a queue reset, yes we can reset a queue if 
failed to suspend it, I am not sure whther
0 is better than guest used idx.

I think we are not able to disable VHOST_DEBUG because customers can 
build QEMU by their own.

Thanks
>
> Thanks
>
>
>     Thanks!
>
>     >     r = dev->vhost_ops->vhost_get_vring_base(dev, &state);
>     >     if (r < 0) {
>     >     ...
>     >     }.else {
>     >         virtio_queue_set_last_avail_idx(vdev, idx, state.num);
>     >     }
>     >
>     > Thanks
>     >
>     > > Then vhost will print an error shows something failed.
>     > >
>     > > The error msg is confused, as stated in the commit log, restoring
>     > > last_avail_idx with guest used idx
>     > > is a workaround rather than a failure. And no needs to fetch
>     last_avail_idx
>     > > when power off.
>     > >
>     > > Thanks
>     > > >
>     > > > Thanks
>     > > >
>     > > >> +    }
>     > > >> +
>     > > >>       if (v->shadow_vqs_enabled) {
>     > > >>           ring->num =
>     virtio_queue_get_last_avail_idx(dev->vdev, ring->index);
>     > > >>           return 0;
>     > > >> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
>     > > >> index 82394331bf..7dd90cff3a 100644
>     > > >> --- a/hw/virtio/vhost.c
>     > > >> +++ b/hw/virtio/vhost.c
>     > > >> @@ -1262,7 +1262,7 @@ void vhost_virtqueue_stop(struct
>     vhost_dev *dev,
>     > > >>
>     > > >>       r = dev->vhost_ops->vhost_get_vring_base(dev, &state);
>     > > >>       if (r < 0) {
>     > > >> -        VHOST_OPS_DEBUG(r, "vhost VQ %u ring restore
>     failed: %d", idx, r);
>     > > >> +        VHOST_OPS_DEBUG(r, "sync last avail idx to the
>     guest used idx for vhost VQ %u", idx);
>     > > >>           /* Connection to the backend is broken, so let's
>     sync internal
>     > > >>            * last avail idx to the device used idx.
>     > > >>            */
>     > > >> --
>     > > >> 2.39.3
>     > > >>
>     > >
>     >
>

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