Re: [Qemu-devel] [RFC 24/29] vhost+postcopy: Lock around set_mem_table

[Maxime Coquelin]

On 06/28/2017 09:00 PM, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert"<address@hidden>
>
> **HACK - better solution needed **
> We have the situation where:
>
>       qemu                      bridge
>
>       send set_mem_table
>                                map memory
>    a)                          mark area with UFD
>                                send reply with map addresses
>    b)                          start using
>    c) receive reply
>
>    As soon as (a) happens qemu might start seeing faults
> from memory accesses (but doesn't until b); but it can't
> process those faults until (c) when it's received the
> mmap addresses.
>
> Make the fault handler spin until it gets the reply in (c).
>
> At the very least this needs some proper locks, but preferably
> we need to split the message.

Yes, maybe the slave channel could be used to send the ufds with
a dedicated request? The backend would set the reply-ack flag, so that
it starts accessing the guest memory only when Qemu is ready to handle
faults.

Note that the slave channel support has not been implemented in Qemu's
libvhost-user yet, but this is something I can do if we feel the need.

Maxime