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SubjectRe: [PATCH 1/5] KVM: x86: avoid atomic operations on APICv vmentry
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);
^^
bad whitespace.

> +}

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.

> +
> static inline int pi_test_on(struct pi_desc *pi_desc)
> {
> return test_bit(POSTED_INTR_ON,

Other than that,

Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>

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