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

[Stefan Hajnoczi]

On Tue, Jul 20, 2021 at 01:32:43PM +0200, Kevin Wolf wrote:
> 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?

Yes. I think so. I'll investigate.

Stefan

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