Unlike the kernel, vhost-user backend knows nothing about QEMU's
userspace addresses. We can pass GPA instead and nothing changes.
Generally, vhost-user servers need these addresses only to calculate
offsets inside memory regions. Still, some servers (QEMU's internal
is one example) may do checks for passed addresses to be "userspace
addresses", for example check for non-zero. That's why we need
additional negotiation for the feature.
The benefit: this opens the doors for further implementation of
local migration (live-update) with passing open vhost-related FDs
through UNIX domain socket. This way the connection with backend is
kept live and untouched.
Without this change, we would have to communicate with backend to
inform it about UVA address changes, but it's better to simply use
more stable GPA numbers, which don't change after migration.
Additionally, the current implementation exposes QEMU's process
address space by passing UVA, which breaks ASLR. New protocol
feature avoids that.
Note, that we do nothing with backend messages and replies.
Frontends have to work with backends userspace addresses anyway,
because they come from userfaultfd.
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Based-on: <20260206095258.894504-1-vsementsov@yandex-team.ru>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20260420202032.714884-11-vsementsov@yandex-team.ru>
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