linux-kernel - Re: kvm vmload/vmsave vs tss.ist

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Date:	Thu, 25 Dec 2008 20:19:58 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Joerg Roedel <joerg.roedel@....com>,
	Benjamin Serebrin <benjamin.serebrin@....com>,
	linux-kernel <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org,
	Alexander Graf <agraf@...e.de>,
	Arjan van de Ven <arjan@...radead.org>,
	Alexander van Heukelum <heukelum@...tmail.fm>
Subject: Re: kvm vmload/vmsave vs tss.ist

Avi Kivity wrote:
>
> I'm suggesting
>
>    check irqcount
>    if (wasnt_in_irq)
>        rsp = irqstack
>    ++irqcount
>
> If the NMI happens before the increment, we'll switch the stack 
> unconditionally, and if the NMI happens after the increment, then we 
> won't switch the stack, but we're guaranteed to be on the irqstack 
> anyway.  The window size is negative :)
>
> Similarly, the exit path should be
>
>    oldstack_reg = oldstack;
>    --irqcount;
>    rsp = oldstack_register;
>
> To guarantee that by the time we decrement irqcount, we don't need the 
> stack anymore.
>

On the other hand, checking %rsp allows us to drop irqcount completely, 
so maybe it's better.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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