From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ingo Molnar <mingo@elte.hu>
Subject: Re: kvm vmload/vmsave vs tss.ist
Date: Thu, 25 Dec 2008 17:42:32 +0100
Message-ID: <20081225164232.GA25195@elte.hu>
References: <49539FD0.7070103@redhat.com> <20081225151757.GA25117@elte.hu> <4953AAE5.4000708@redhat.com> <20081225162107.GB14486@elte.hu>
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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: Avi Kivity <avi@redhat.com>
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* Ingo Molnar <mingo@elte.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