Re: [PATCH 3/5] KVM: PPC: Book3S HV: Handle memory slot deletion and modification correctly

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, Aug 24, 2012 at 07:29:53PM +1000, Paul Mackerras wrote:
> On Thu, Aug 23, 2012 at 10:55:37AM -0300, Marcelo Tosatti wrote:
> 
> > There are a number of options to consider:
> > 
> > 1) Merge the current patchset from Paul, which has two downsides:
> > 	1-1) It contains an unfixable race.
> 
> The race being that new HTPEs using the old base address could get
> inserted after we flush the old HPTEs and before we update the memslot
> array?

Yes. That race, for the slot deletion case, is fixed by RCU assignment 
of memslot marked with KVM_MEMSLOT_INVALID.

For the base change case, x86 flushes all translations via the
kvm_arch_flush_shadow() at the of __kvm_set_memory. 

Which is not enough for PPC since you need to flush _before_ new
memslot is visible.

> I think we could solve that one by temporarily marking the memslot
> invalid for changes of base_gfn as well as for memslot deletions.  Can
> you see any problem with that?  It means that guest accesses to the
> old memslot addresses would trap or fail, but if the guest is trying
> to access a device while its BAR is being changed, then I think it
> deserves that.

No, i don't see any problem with that. Sent a patchset.