linux-kernel - Re: [PATCH 0/3] Remove interrupt stack table usage from x86

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Message-ID: <4957E17A.9080203@redhat.com>
Date:	Sun, 28 Dec 2008 22:28:42 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Ingo Molnar <mingo@...e.hu>, x86@...nel.org,
	linux-kernel <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>
Subject: Re: [PATCH 0/3] Remove interrupt stack table usage from x86_64 kernel

Andi Kleen wrote:
>> This makes the whole thing unworthwhile.  The vmload/vmsave pair costs 
>> only 200 cycles (I should have started with this), and 120 cycles on the 
>> heavyweight path plus complexity are not worth 200 cycles on the 
>> lightweight path.
>>     
>
> Actually to switch ISTs you need to change the TSS, not the IDT.
> But I suppose that won't be any faster.
>   

I can't touch the TSS (that's the starting point of the exercise).  The 
plan was to have a copy of the IDT with all IST pointers zeroed out (or 
to have per-cpu IDT and zero out the IST pointers when entering guest 
mode, restoring them on context switch).

It's not worth it though.

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

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