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Subject: Re: [PATCH 1/5] KVM: x86: avoid atomic operations on APICv vmentry
From: Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <1476469291-5039-2-git-send-email-pbonzini@redhat.com>
Date: Fri, 14 Oct 2016 11:50:54 -0700
Cc: LKML <linux-kernel@vger.kernel.org>, KVM <kvm@vger.kernel.org>,
        =?utf-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Yang Zhang <yang.zhang.wz@gmail.com>, feng.wu@intel.com,
        mst@redhat.com
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To: Paolo Bonzini <pbonzini@redhat.com>
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> 
> 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?