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From: Peter Zijlstra <peterz@infradead.org>
To: Sean Christopherson <seanjc@google.com>
Cc: Li RongQing <lirongqing@baidu.com>,
	pbonzini@redhat.com, vkuznets@redhat.com, wanpengli@tencent.com,
	jmattson@google.com, tglx@linutronix.de, bp@alien8.de,
	x86@kernel.org, kvm@vger.kernel.org, joro@8bytes.org
Subject: Re: [PATCH][v3] KVM: x86: refine kvm_vcpu_is_preempted
Date: Wed, 2 Feb 2022 23:17:03 +0100	[thread overview]
Message-ID: <20220202221703.GE20638@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <Yfq19FSnASMfd0BH@google.com>

On Wed, Feb 02, 2022 at 04:48:52PM +0000, Sean Christopherson wrote:

> > > +"movb	" __stringify(KVM_STEAL_TIME_preempted) "+steal_time(%rax), %al;"
> > > +"andl	$" __stringify(KVM_VCPU_PREEMPTED) ", %eax;"
> > > +"setnz	%al;"
> > 
> > Isn't the below the simpler way of writing that same?
> 
> The AND is needed to clear RAX[63:8], e.g. to avoid breakage if the return value
> is changed from a bool to something larger, or the compiler uses more than a byte
> for _Bool.

While C doesn't specify _Bool the arch ABI most certainly does, and
x86_64 ABI guarantees a byte there, changing the return type is the only
possible option here.

Anyway, I don't mind the patch and the proposed Changelog does clarify
that this is an exercise in paranoia.

  reply	other threads:[~2022-02-02 22:17 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-01-17  5:37 [PATCH][v3] KVM: x86: refine kvm_vcpu_is_preempted Li RongQing
2022-02-02 14:54 ` Peter Zijlstra
2022-02-02 16:48   ` Sean Christopherson
2022-02-02 22:17     ` Peter Zijlstra [this message]
2022-02-06 11:26     ` 答复: " Li,Rongqing

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