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

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Message-ID: <20140731174359.GI26853@arm.com>
Date:	Thu, 31 Jul 2014 18:44:00 +0100
From:	Will Deacon <will.deacon@....com>
To:	Peter Maydell <peter.maydell@...aro.org>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	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>,
	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 Thu, Jul 31, 2014 at 06:36:35PM +0100, Peter Maydell wrote:
> 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.

(and in case that wasn't clear, local_irq_disable() doesn't prevent an
interrupt from waking you up from wfi, otherwise our idle code would be
broken).

Will
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