linux-kernel - Re: [PATCH] x86, nmi: workaround sti; hlt race vs nmi; intr

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Date:	Mon, 27 Sep 2010 11:13:12 +0200
From:	Alexander Graf <agraf@...e.de>
To:	Avi Kivity <avi@...hat.com>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] x86, nmi: workaround sti; hlt race vs nmi; intr


On 27.09.2010, at 10:38, Avi Kivity wrote:

> On 09/19/2010 06:28 PM, Avi Kivity wrote:
>> On machines without monitor/mwait we use an sti; hlt sequence to atomically
>> enable interrupts and put the cpu to sleep.  The sequence uses the "interrupt
>> shadow" property of the sti instruction: interrupts are enabled only after
>> the instruction following sti has been executed.  This means an interrupt
>> cannot happen in the middle of the sequence, which would leave us with
>> the interrupt processed but the cpu halted.
>> 
>> The interrupt shadow, however, can be broken by an nmi; the following
>> sequence
>> 
>>    sti
>>      nmi ... iret
>>      # interrupt shadow disabled
>>      intr ... iret
>>    hlt
>> 
>> puts the cpu to sleep, even though the interrupt may need additional processing
>> after the hlt (like scheduling a task).
>> 
>> sti is explicitly documented not to force an interrupt shadow; though many
>> processors do inhibit nmi immediately after sti.
>> 
>> Avoid the race by checking, during an nmi, if we hit the safe halt sequence.
>> If we did, increment the instruction pointer past the hlt instruction; this
>> allows an immediately following interrupt to return to a safe place.
>> 
>> Signed-off-by: Avi Kivity<avi@...hat.com>
> 
> Ping.

Wow, this is incredibly ugly :). Can't we just mask NMIs when the interrupt shadow is active?


Alex

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