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

[Paolo Bonzini <pbonzini@redhat.com>]

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.

Paolo