Re: [Qemu-devel] qemu-kvm aborts - vhost_dev_unassign_memory: Assertion `to >= 0' failed.

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Aug 03, 2011 at 02:48:05PM +0300, Avi Kivity wrote:
> On 08/01/2011 08:44 PM, David Ahern wrote:
> >qemu-kvm.git as of:
> >
> >commit dacdc4b10bafbb21120e1c24a9665444768ef999
> >Merge: 7b69d4f 0af4922
> >Author: Avi Kivity<avi@redhat.com>
> >Date:   Sun Jul 31 11:42:26 2011 +0300
> >
> >     Merge branch 'upstream-merge' into next
> >
> >is aborting with the error:
> >
> >qemu-kvm: qemu-kvm.git/hw/vhost.c:123: vhost_dev_unassign_memory:
> >Assertion `to>= 0' failed.
> >Aborted
> >
> 
> It's a bug in vhost:
> 
> /* Assign/unassign. Keep an unsorted array of non-overlapping
>  * memory regions in dev->mem. */
> static void vhost_dev_unassign_memory(struct vhost_dev *dev,
>                                       uint64_t start_addr,
>                                       uint64_t size)
> {
>     int from, to, n = dev->mem->nregions;
>     /* Track overlapping/split regions for sanity checking. */
>     int overlap_start = 0, overlap_end = 0, overlap_middle = 0, split = 0;
> 
>     for (from = 0, to = 0; from < n; ++from, ++to) {
>         struct vhost_memory_region *reg = dev->mem->regions + to;
>         uint64_t reglast;
>         uint64_t memlast;
>         uint64_t change;
> 
>         /* clone old region */
>         if (to != from) {
>             memcpy(reg, dev->mem->regions + from, sizeof *reg);
>         }
> 
>         /* No overlap is simple */
>         if (!ranges_overlap(reg->guest_phys_addr, reg->memory_size,
>                             start_addr, size)) {
>             continue;
>         }
> 
>         /* Split only happens if supplied region
>          * is in the middle of an existing one. Thus it can not
>          * overlap with any other existing region. */
>         assert(!split);
> 
>         reglast = range_get_last(reg->guest_phys_addr, reg->memory_size);
>         memlast = range_get_last(start_addr, size);
> 
>         /* Remove whole region */
>         if (start_addr <= reg->guest_phys_addr && memlast >= reglast) {
>             --dev->mem->nregions;
>             --to;
>             assert(to >= 0);
>             ++overlap_middle;
>             continue;
>         }
> 
> 
> We're removing the first region, and 'to' goes negative.  Michael?

Yes, that assert is wrong.

--->
Subject: vhost: remove an incorrect assert

The 'to' can go negative when the first region gets removed
(it gets incremented by to 0 immediately afterward), which
makes the assertion fail. Nothing breaks if
to < 0 here so just remove the assert.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

----

diff --git a/hw/vhost.c b/hw/vhost.c
index c3d8821..19e7255 100644
--- a/hw/vhost.c
+++ b/hw/vhost.c
@@ -120,7 +120,6 @@ static void vhost_dev_unassign_memory(struct vhost_dev *dev,
         if (start_addr <= reg->guest_phys_addr && memlast >= reglast) {
             --dev->mem->nregions;
             --to;
-            assert(to >= 0);
             ++overlap_middle;
             continue;
         }