Re: [PATCH RFC 00/15] vhost-user: isolated memory

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Sat, Jul 25, 2026 at 12:45:08PM +0900, Akihiko Odaki wrote:
> I also hope this could fix vhost-user for TCG:
> https://gitlab.com/qemu-project/qemu/-/work_items/2092
> 
> Perhaps the memory isolation should be enabled by default for TCG.

If I understand the bug correctly, TCG needs the dirty bitmap to be
up-to-date and vhost-user's dirty logging mechanism does not suffice.
Memory isolation would keep the dirty bitmap up-to-date but copying data
between guest RAM and the isolated memory would add overhead.

For the TCG use case it should be possible to use just
vhost-shadow-virtqueue.c without isolated memory so that
vhost_svq_flush() -> virtqueue_fill() -> virtqueue_unmap_sg() ->
dma_memory_unmap() keeps the dirty bitmap updated. There is still an
overhead because the virtqueue kick/call is intercepted by QEMU, but
there is no memcpy.

The TCG use case is something that could be tackled once memory
isolation is finished. If no one has time to implement the pure svq
solution for TCG, then memory isolation could be used in any case.

Stefan

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