Re: [PATCH] KVM: MMU: Inform users of mmio generation wraparound

[Gleb Natapov <gleb@redhat.com>]

On Thu, Jun 20, 2013 at 12:59:54PM +0200, Paolo Bonzini wrote:
> Il 20/06/2013 10:59, Takuya Yoshikawa ha scritto:
> > Without this information, users will just see unexpected performance
> > problems and there is little chance we will get good reports from them:
> > note that mmio generation is increased even when we just start, or stop,
> > dirty logging for some memory slot, in which case users should never
> > expect all shadow pages to be zapped.
> > 
> > Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
> > ---
> >  arch/x86/kvm/mmu.c |    4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> > index c60c5da..bc8302f 100644
> > --- a/arch/x86/kvm/mmu.c
> > +++ b/arch/x86/kvm/mmu.c
> > @@ -4385,8 +4385,10 @@ void kvm_mmu_invalidate_mmio_sptes(struct kvm *kvm)
> >  	 * The max value is MMIO_MAX_GEN - 1 since it is not called
> >  	 * when mark memslot invalid.
> >  	 */
> > -	if (unlikely(kvm_current_mmio_generation(kvm) >= (MMIO_MAX_GEN - 1)))
> > +	if (unlikely(kvm_current_mmio_generation(kvm) >= (MMIO_MAX_GEN - 1))) {
> > +		printk(KERN_INFO "kvm: zapping shadow pages for mmio generation wraparound");
> 
> This should at least be rate-limited, because it is guest triggerable.
> 
It will be hard for guest to triggers it 1 << 19 times too fast though.

> But why isn't the kvm_mmu_invalidate_zap_all_pages tracepoint enough?
> 
This one will trigger during slot deletion/move too.

I would put it in to see if it actually triggers in some real world
workloads (skipping the firs wraparound since it is intentional),
we can always drop it if it will turn out to create a lot of noise.
 
--
			Gleb.