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

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Message-Id: <4B0C832B-BB75-40BD-85A9-9DC84DEB44E2@gmail.com>
Date:   Fri, 14 Oct 2016 11:50:54 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Yang Zhang <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

> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 23b99f305382..63a442aefc12 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -342,9 +342,11 @@ void __kvm_apic_update_irr(u32 *pir, void *regs)
> 	u32 i, pir_val;
> 
> 	for (i = 0; i <= 7; i++) {
> -		pir_val = xchg(&pir[i], 0);
> -		if (pir_val)
> +		pir_val = READ_ONCE(pir[i]);

Out of curiosity, do you really need this READ_ONCE?