From mboxrd@z Thu Jan  1 00:00:00 1970
From: Avi Kivity <avi@redhat.com>
Subject: Re: kvm vmload/vmsave vs tss.ist
Date: Thu, 25 Dec 2008 20:19:58 +0200
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References: <49539FD0.7070103@redhat.com> <20081225151757.GA25117@elte.hu> <4953AAE5.4000708@redhat.com> <20081225162107.GB14486@elte.hu> <20081225164232.GA25195@elte.hu> <4953C57F.70502@redhat.com> <20081225175821.GA23390@elte.hu> <4953CD1A.5090906@redhat.com>
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Cc: "H. Peter Anvin" <hpa@zytor.com>,
	Joerg Roedel <joerg.roedel@amd.com>,
	Benjamin Serebrin <benjamin.serebrin@amd.com>,
	linux-kernel <linux-kernel@vger.kernel.org>, kvm@vger.kernel.org,
	Alexander Graf <agraf@suse.de>,
	Arjan van de Ven <arjan@infradead.org>,
	Alexander van Heukelum <heukelum@fastmail.fm>
To: Ingo Molnar <mingo@elte.hu>
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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.