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

From: Michael S. Tsirkin
Date: Thu Jun 18 2015 - 10:47:49 EST

On Thu, Jun 18, 2015 at 03:46:14PM +0200, Paolo Bonzini wrote:
> On 18/06/2015 15:19, Michael S. Tsirkin wrote:
> > 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.
> You're right, the problem isn't directly related to RCU. RCU would be
> easy to handle by using synchronize_rcu instead of call_rcu. While I
> identified an RCU-related problem with Igor's patches, it's much more
> entrenched.
> RAM can be used by asynchronous operations while the VM runs, between
> address_space_map and address_space_unmap. It is possible and common to
> have a quiescent state between the map and unmap, and a memory map
> change can happen in the middle of this. Normally this is not a
> problem, because changes to the memory map do not make the hva go away
> (memory regions are reference counted).

Right, so you want mmap(MAP_NORESERVE) when that reference
count becomes 0.

> However, with Igor's patches a memory_region_del_subregion will cause a
> mmap(MAP_NORESERVE), which _does_ have the effect of making the hva go away.
> I guess one way to do it would be to alias the same page in two places,
> one for use by vhost and one for use by everything else. However, the
> kernel does not provide the means to do this kind of aliasing for
> anonymous mmaps.
> Paolo

Basically pages go away on munmap, so won't simple
do the trick?

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