Re: [PATCH 3/5] vhost: support upto 509 memory regions

From: Michael S. Tsirkin
Date: Thu Jun 18 2015 - 09:19:28 EST

On Thu, Jun 18, 2015 at 01:50:32PM +0200, Paolo Bonzini wrote:
> On 18/06/2015 13:41, Michael S. Tsirkin wrote:
> > On Thu, Jun 18, 2015 at 01:39:12PM +0200, Igor Mammedov wrote:
> >> Lets leave decision upto users instead of making them live with
> >> crashing guests.
> >
> > Come on, let's fix it in userspace.
> It's not trivial to fix it in userspace. Since QEMU uses RCU there
> isn't a single memory map to use for a linear gpa->hva map.

Could you elaborate?

I'm confused by this mention of RCU.
You use RCU for accesses to the memory map, correct?
So memory map itself is a write side operation, as such all you need to
do is take some kind of lock to prevent conflicting with other memory
maps, do rcu sync under this lock.

> I find it absurd that we're fighting over 12K of memory.
> Paolo

I wouldn't worry so much if it didn't affect kernel/userspace API.
Need to be careful there.

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