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Date: Wed, 30 Aug 2017 14:50:02 +0800
From: Peter Xu <peterx@redhat.com>
Message-ID: <20170830065002.GA10627@pxdev.xzpeter.org>
References: <20170824192730.8440-1-dgilbert@redhat.com>
	<20170824192730.8440-26-dgilbert@redhat.com>
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Subject: Re: [Qemu-devel] [RFC v2 25/32] vhost+postcopy: Lock around
 set_mem_table
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To: "Dr. David Alan Gilbert (git)" <dgilbert@redhat.com>
Cc: qemu-devel@nongnu.org, maxime.coquelin@redhat.com, a.perevalov@samsung.com, mst@redhat.com, marcandre.lureau@redhat.com, quintela@redhat.com, lvivier@redhat.com, aarcange@redhat.com, felipe@nutanix.com

On Thu, Aug 24, 2017 at 08:27:23PM +0100, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> 
> **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.

I see discussions about slave channel and ack mechanism in previous
post.  So it's still not adopted (which looks doable)?  What's our
further plan?

-- 
Peter Xu