Re: [RFC] vhost: Clear the pending messages on vhost_init_device_iotlb()

Jason Wang
2022-11-08T03:10:54Z

On Tue, Nov 8, 2022 at 7:06 AM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Mon, Nov 07, 2022 at 10:10:06PM +0100, Eric Auger wrote:
> > Hi Michael,
> > On 11/7/22 21:42, Michael S. Tsirkin wrote:
> > > On Mon, Nov 07, 2022 at 09:34:31PM +0100, Eric Auger wrote:
> > >> When the vhost iotlb is used along with a guest virtual iommu
> > >> and the guest gets rebooted, some MISS messages may have been
> > >> recorded just before the reboot and spuriously executed by
> > >> the virtual iommu after the reboot. Despite the device iotlb gets
> > >> re-initialized, the messages are not cleared. Fix that by calling
> > >> vhost_clear_msg() at the end of vhost_init_device_iotlb().
> > >>
> > >> Signed-off-by: Eric Auger <eric.auger@redhat.com>
> > >> ---
> > >> drivers/vhost/vhost.c | 1 +
> > >> 1 file changed, 1 insertion(+)
> > >>
> > >> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > >> index 40097826cff0..422a1fdee0ca 100644
> > >> --- a/drivers/vhost/vhost.c
> > >> +++ b/drivers/vhost/vhost.c
> > >> @@ -1751,6 +1751,7 @@ int vhost_init_device_iotlb(struct vhost_dev *d, bool enabled)
> > >> }
> > >>
> > >> vhost_iotlb_free(oiotlb);
> > >> + vhost_clear_msg(d);
> > >>
> > >> return 0;
> > >> }
> > > Hmm. Can't messages meanwhile get processes and affect the
> > > new iotlb?
> > Isn't the msg processing stopped at the moment this function is called
> > (VHOST_SET_FEATURES)?
> >
> > Thanks
> >
> > Eric
>
> It's pretty late here I'm not sure. You tell me what prevents it.

So the proposed code assumes that Qemu doesn't process device IOTLB
before VHOST_SET_FEAETURES. Consider there's no reset in the general
vhost uAPI, I wonder if it's better to move the clear to device code
like VHOST_NET_SET_BACKEND. So we can clear it per vq?

>
> BTW vhost_init_device_iotlb gets enabled parameter but ignores
> it, we really should drop that.

Yes.

>
> Also, it looks like if features are set with VIRTIO_F_ACCESS_PLATFORM
> and then cleared, iotlb is not properly cleared - bug?

Not sure, old IOTLB may still work. But for safety, we need to disable
device IOTLB in this case.

Thanks

>
>
> > >
> > >
> > >> --
> > >> 2.37.3
>