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

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Message-ID: <20081225164232.GA25195@elte.hu>
Date:	Thu, 25 Dec 2008 17:42:32 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Avi Kivity <avi@...hat.com>
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 <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.

	Ingo
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