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

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Date:   Wed, 26 Oct 2016 21:53:45 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        yang.zhang.wz@...il.com, feng.wu@...el.com, mst@...hat.com
Subject: Re: [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@...hat.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@...hat.com>