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

[Gleb Natapov <gleb@redhat.com>]

On Thu, Jun 20, 2013 at 02:35:36PM +0200, Paolo Bonzini wrote:
> Il 20/06/2013 13:45, Gleb Natapov ha scritto:
> > 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.
> 
> Reading a ROM in a loop can trigger it in less than 5 minutes on my
> machine.  Not a lot of noise, but enough to be annoying.  I think the
> existing tracepoint is enough, or we can add a more specific one here.
> 
5 minutes after first wraparound?

--
			Gleb.