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

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Message-ID: <4953C57F.70502@redhat.com>
Date:	Thu, 25 Dec 2008 19:40:15 +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

Ingo Molnar wrote:
> * Ingo Molnar <mingo@...e.hu> wrote:
>
>   
>> i'd suggest to reuse the irq-stacks for this. Right now on 64-bit we've 
>> got the following stack layout: 8K process stacks, a 16K IRQ stack on 
>> each CPU, shared by all IRQs. Then we have the IST stacks with weird 
>> sizes: debug:8K, the others: 4K.
>>     
>
> this has to be done carefully though, as there's a subtle detail here: 
> right now the pda_irqcount and the pda_irqstackptr logic in entry_64.S is 
> not re-entry safe and relies on IRQs being off.
>
> If critical exceptions are moved to the IRQ stack then %rsp switching to 
> the IRQ stack has to be done atomically: instead of using the pda_irqcount 
> check the %rsp value itself should be checked against pda_irqstackptr - if 
> it's within that 16K range then we are already on the IRQ stack and do not 
> need to switch to it but can just use the current %rsp.
>   

I think it's enough to switch %rsp before incrementing irqcount, no?

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

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