Re: [PATCH 1/5] KVM: x86: avoid atomic operations on APICv vmentry

["Radim Krčmář" <rkrcmar@redhat.com>]

2016-10-27 19:51+0300, Michael S. Tsirkin:
> On Thu, Oct 27, 2016 at 06:44:00PM +0200, Radim Krčmář wrote:
>> 2016-10-27 00:42+0300, Michael S. Tsirkin:
>> > On Wed, Oct 26, 2016 at 09:53:45PM +0200, Radim Krčmář wrote:
>> >> 2016-10-14 20:21+0200, Paolo Bonzini:
>> >> > On some benchmarks (e.g. netperf with ioeventfd disabled), APICv
>> >> > posted interrupts turn out to be slower than interrupt injection via
>> >> > KVM_REQ_EVENT.
>> >> > 
>> >> > This patch optimizes a bit the IRR update, avoiding expensive atomic
>> >> > operations in the common case where PI.ON=0 at vmentry or the PIR vector
>> >> > is mostly zero.  This saves at least 20 cycles (1%) per vmexit, as
>> >> > measured by kvm-unit-tests' inl_from_qemu test (20 runs):
>> >> > 
>> >> >               | enable_apicv=1  |  enable_apicv=0
>> >> >               | mean     stdev  |  mean     stdev
>> >> >     ----------|-----------------|------------------
>> >> >     before    | 5826     32.65  |  5765     47.09
>> >> >     after     | 5809     43.42  |  5777     77.02
>> >> > 
>> >> > Of course, any change in the right column is just placebo effect. :)
>> >> > The savings are bigger if interrupts are frequent.
>> >> > 
>> >> > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>> >> > ---
>> >> > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> >> > @@ -521,6 +521,12 @@ static inline void pi_set_sn(struct pi_desc *pi_desc)
>> >> >  			(unsigned long *)&pi_desc->control);
>> >> >  }
>> >> >  
>> >> > +static inline void pi_clear_on(struct pi_desc *pi_desc)
>> >> > +{
>> >> > +	clear_bit(POSTED_INTR_ON,
>> >> > +  		  (unsigned long *)&pi_desc->control);
>> >> > +}
>> >> 
>> >> We should add an explicit smp_mb__after_atomic() for extra correctness,
>> >> because clear_bit() does not guarantee a memory barrier and we must make
>> >> sure that pir reads can't be reordered before it.
>> >> x86 clear_bit() currently uses locked instruction, though.
>> > 
>> > smp_mb__after_atomic is empty on x86 so it's
>> > a documentation thing, not a correctness thing anyway.
>> 
>> All atomics currently contain a barrier, but the code is also
>> future-proofing, not just documentation: implementation of clear_bit()
>> could drop the barrier and smp_mb__after_atomic() would then become a
>> real barrier.
>> 
>> Adding dma_mb__after_atomic() would be even better as this bug could
>> happen even on a uniprocessor with an assigned device, but people who
>> buy a SMP chip to run a UP kernel deserve it.
> 
> Not doing dma so does not seem to make sense ...

IOMMU does -- it writes to the PIR and sets ON asynchronously.

> Why do you need a barrier on a UP kernel?

If pi_clear_on() doesn't contain a memory barrier (possible future),
then we have the following race: (pir[0] begins as 0.)

    KVM                           |  IOMMU
   -------------------------------+-------------
   pir_val = ACCESS_ONCE(pir[0])  |
                                  | pir[0] = 123
                                  | pi_set_on()
   pi_clear_on()                  |
   if (pir_val)                   |

ACCESS_ONCE() does not prevent the CPU to prefetch pir[0] (ACCESS_ONCE
does nothing in this patch), so if there was 0 in pir[0] before IOMMU
wrote to it, then our optimization to avoid the xchg would yield a false
negative and the interrupt would be lost.