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

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Date:	Mon, 27 Sep 2010 12:31:29 +0200
From:	Joerg Roedel <joro@...tes.org>
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 Sun, Sep 19, 2010 at 06:28:19PM +0200, 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).

Doesn't the interrupt return path check for a re-schedule condition
before iret? So to my believe the handler would not jump back to the
idle task if something else becomes running in the interrupt handler,
no?

	Joerg

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