linux-kernel - Re: [PATCH] arm64: KVM: export current vcpu->pause state via pseudo regs

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Message-ID: <CAFEAcA8RjmjoaEpjoDE5YJdKagtZYZe9TpCdsRPf86W=WCxCsA@mail.gmail.com>
Date:	Thu, 31 Jul 2014 18:36:35 +0100
From:	Peter Maydell <peter.maydell@...aro.org>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Alex Bennée <alex.bennee@...aro.org>,
	"kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
	arm-mail-list <linux-arm-kernel@...ts.infradead.org>,
	kvm-devel <kvm@...r.kernel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	open list <linux-kernel@...r.kernel.org>,
	Gleb Natapov <gleb@...nel.org>
Subject: Re: [PATCH] arm64: KVM: export current vcpu->pause state via pseudo regs

On 31 July 2014 18:21, Paolo Bonzini <pbonzini@...hat.com> wrote:
> What does ARM do if you have a WFI while interrupts are disabled?  On
> x86 after "cli;hlt" only an NMI will wake you up.  With spurious
> wakeups, it's pretty much guaranteed that you will break such "cli;hlt"
> sequences.

The architecture mandates some things that *must* wake you from
a WFI, but it also allows wakeups for other reasons not listed, or
for no reason at all. It's perfectly valid to implement WFI as a NOP
(though it would not be very good for power efficiency, obviously).
Guests which don't surround WFI with a "check whether we should
just go back to WFI" loop are buggy.

thanks
-- PMM
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