On Wed, Sep 16, 2026 at 10:59:03AM -0400, Stefan Hajnoczi wrote: > On Wed, Sep 09, 2026 at 03:59:21PM +0300, Alex Fishman wrote: > > Virtio-mem dynamic memslots are intentionally exposed as separate memory > > regions. If a guest places a vring part across two adjacent slots, a later > > memory update fails vhost ring verification because the complete ring part > > is not contained in one vhost memory region. > > > > Coalesce coherent adjacent sections only when a descriptor table, > > available ring, or used ring crosses their boundary. Unrelated > > unmergeable sections remain separate so they can still be updated > > independently. > > > > Patch 2 adds an end-to-end qtest reproducer using a fake vhost-user > > backend and virtio-mem dynamic memslots. It also verifies that a newly > > activated slot is not folded into the merge required by the crossing > > vring. > > > > Tested with the vhost-user/mem-slots/boundary qtest. > > > > Buglink: https://redhat.atlassian.net/browse/RHEL-146583 > > > > Alex Fishman (2): > > vhost: coalesce unmergeable sections across vring boundaries > > tests/qtest: Add vhost-user memslot boundary test > > > > hw/virtio/vhost.c | 72 ++++++- > > tests/qtest/vhost-user-test.c | 395 +++++++++++++++++++++++++++++++++- > > 2 files changed, 452 insertions(+), 15 deletions(-) > > > > > > base-commit: ff1d2d19d7e24893e2012d879f8e73077e17b9bd > > -- > > 2.52.0 > > I'm pretty sure a similar problem exists with hw/virtio/virtio.c's > MemoryRegionCaches. If a vring spans virtio-mem dynamic memslots, then > the MemoryRegionCache won't work because the memory range crosses > MemoryRegions.
IIUC address_space_cache_init does not check unmergeable, so virtio doesn't have this problem. > Is it worth looking at that first (e.g. triggering the issue with a test > case that uses a built-in VIRTIO device instead of a vhost-user device) > before implementing a solution that is specific to vhost-user? > > Stefan
