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

[Igor Mammedov]

On Thu, 18 Jun 2015 16:47:33 +0200
"Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:

> 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
> 	lock
> 	munmap
> 	mmap(MAP_NORESERVE)
> 	unlock
> do the trick?
at what time are you suggesting to do this?



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