Re: [PATCH] vhost: Don't special case vq->used_phys in vhost_get_log_size()

[Greg Kurz <groug@kaod.org>]

On Sat, 10 Oct 2020 11:30:28 +0800
Jason Wang <jasowang@redhat.com> wrote:

> 
> On 2020/10/8 上午12:30, Greg Kurz wrote:
> > The first loop in vhost_get_log_size() computes the size of the dirty log
> > bitmap so that it allows to track changes in the entire guest memory, in
> > terms of GPA.
> >
> > When not using a vIOMMU, the address of the vring's used structure,
> > vq->used_phys, is a GPA. It is thus already covered by the first loop.
> >
> > When using a vIOMMU, vq->used_phys is a GIOVA that will be translated
> > to an HVA when the vhost backend needs to update the used structure. It
> > will log the corresponding GPAs into the bitmap but it certainly won't
> > log the GIOVA.
> >
> > So in any case, vq->used_phys shouldn't be explicitly used to size the
> > bitmap. Drop the second loop.
> >
> > This fixes a crash of the source when migrating a guest using in-kernel
> > vhost-net and iommu_platform=on on POWER, because DMA regions are put
> > over 0x800000000000000ULL. The resulting insanely huge log size causes
> > g_malloc0() to abort.
> >
> > BugLink: https://bugzilla.redhat.com/show_bug.cgi?id=1879349
> > Signed-off-by: Greg Kurz <groug@kaod.org>
> > ---
> >
> > This supersedes "vhost: Ignore vrings in dirty log when using a vIOMMU"
> >
> > http://patchwork.ozlabs.org/project/qemu-devel/patch/160105498386.68108.2145229309875282336.stgit@bahia.lan/
> > ---
> >   hw/virtio/vhost.c |   10 ----------
> >   1 file changed, 10 deletions(-)
> >
> > diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> > index 011951625442..c02b658b597f 100644
> > --- a/hw/virtio/vhost.c
> > +++ b/hw/virtio/vhost.c
> > @@ -172,16 +172,6 @@ static uint64_t vhost_get_log_size(struct vhost_dev *dev)
> >                                          reg->memory_size);
> >           log_size = MAX(log_size, last / VHOST_LOG_CHUNK + 1);
> >       }
> > -    for (i = 0; i < dev->nvqs; ++i) {
> > -        struct vhost_virtqueue *vq = dev->vqs + i;
> > -
> > -        if (!vq->used_phys && !vq->used_size) {
> > -            continue;
> > -        }
> > -
> > -        uint64_t last = vq->used_phys + vq->used_size - 1;
> > -        log_size = MAX(log_size, last / VHOST_LOG_CHUNK + 1);
> > -    }
> >       return log_size;
> >   }
> >   
> 
> 
> Acked-by: Jason Wang <jasowang@redhat.com>
> 

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