Re: [PATCH RFC] KVM: MMU: Don't use RCU for lockless shadow walking

[Avi Kivity <avi@redhat.com>]

On 04/24/2012 12:24 PM, Avi Kivity wrote:
> On 04/24/2012 04:17 AM, Marcelo Tosatti wrote:
> > On Mon, Apr 23, 2012 at 07:16:52PM +0300, Avi Kivity wrote:
> > > Using RCU for lockless shadow walking can increase the amount of memory
> > > in use by the system, since RCU grace periods are unpredictable.  We also
> > > have an unconditional write to a shared variable (reader_counter), which
> > > isn't good for scaling.
> > > 
> > > Replace that with a scheme similar to x86's get_user_pages_fast(): disable
> > > interrupts during lockless shadow walk to force the freer
> > > (kvm_mmu_commit_zap_page()) to wait for the TLB flush IPI to find the
> > > processor with interrupts enabled.
> > > 
> > > We also add a new vcpu->mode, READING_SHADOW_PAGE_TABLES, to prevent
> > > kvm_flush_remote_tlbs() from avoiding the IPI.
> > > 
> >
> > Should add an explicit mb after prepare_zap_page? (currently rely on
> > unrelated ones internal to flush_remote_tlbs).
>
> Yes.

Is that really true?  If unlocked shadow walk doesn't see the changes by
prepare_zap_page(), it will still see valid sptes that will not be
reused (blocked by kvm_flush_remote_tlbs()).  If it does see the
changes, it won't access those sptes at all. Either way we're fine.

-- 
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