On Tue, Jul 11, 2023 at 9:05 AM Jason Wang <[email protected]> wrote:
>
> On Tue, Jul 11, 2023 at 12:09 PM Zhu, Lingshan <[email protected]> wrote:
> >
> >
> >
> > On 7/11/2023 10:50 AM, Jason Wang wrote:
> > > On Mon, Jul 10, 2023 at 4:53 PM Zhu Lingshan <[email protected]>
> > > 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 <[email protected]>
> > >> ---
> > >> 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.
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).
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
> > >>
> >
>
