Re: [PATCH v3 0/3] vhost-user: warn when guest RAM is not shared

[Kevin Wolf <kwolf@redhat.com>]

Am 14.07.2021 um 11:29 hat Stefan Hajnoczi geschrieben:
> v3:
>  * Rebased
> v2:
>  * Clarify that object-memory-memfd enables share=on by default [Marc-André]
>  * Rebased
> 
> vhost-user requires -object memory-backend-*,share=on option so that QEMU uses
> mmap(MAP_SHARED) on guest RAM that is shared with the vhost-user device backend
> process. This is needed so the QEMU process sees changes made by the vhost-user
> device backend process, and vice versa.
> 
> Today QEMU and the vhost-user device process will start up and then fail with a
> confusing error message if the user forgot to specify share=on.
> 
> This patch series adds a warning letting the user know that share=on is
> required.

Hmm... This seems to work when I use shared=off for the memory backend,
even though the error message is easy to miss because more error
messages follow:

    $ LANG=C build/qemu-system-x86_64 -object memory-backend-memfd,id=mem,size=1G,share=off -m 1G -M memory-backend=mem -chardev socket,path=/tmp/vhost.sock,id=vhost1 -device vhost-user-blk-pci,chardev=vhost1
    qemu-system-x86_64: -device vhost-user-blk-pci,chardev=vhost1: warning: Found vhost-user memory region without MAP_SHARED (did you forget -object memory-*,share=on?)
    qemu-system-x86_64: Unexpected end-of-file before all data were read
    qemu-system-x86_64: Failed to set msg fds.
    qemu-system-x86_64: vhost VQ 0 ring restore failed: -1: Resource temporarily unavailable (11)

However, if I don't create the memory backend at all (which is probably
the most common error users will make), then the new message isn't
printed:

    $ LANG=C build/qemu-system-x86_64 -m 1G -chardev socket,path=/tmp/vhost.sock,id=vhost1 -device vhost-user-blk-pci,chardev=vhost1
    qemu-system-x86_64: Unexpected end-of-file before all data were read
    qemu-system-x86_64: Failed to set msg fds.
    qemu-system-x86_64: vhost VQ 0 ring restore failed: -1: Resource temporarily unavailable (11)

Wouldn't we want a warning for the second case, too?

Kevin