linux-kernel - Re: [Lguest] [PATCH RFC/RFB] x86_64, i386: interrupt dispatch changes

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Date:	Wed, 26 Nov 2008 23:50:20 +0200
From:	Avi Kivity <avi@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Alexander van Heukelum <heukelum@...tmail.fm>, lguest@...abs.org,
	jeremy@...source.com, LKML <linux-kernel@...r.kernel.org>,
	Mike Travis <travis@....com>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Steven Rostedt <srostedt@...hat.com>,
	Andi Kleen <andi@...stfloor.org>, Ingo Molnar <mingo@...e.hu>,
	Alexander van Heukelum <heukelum@...lshack.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [Lguest] [PATCH RFC/RFB] x86_64,	i386: interrupt dispatch changes

Avi Kivity wrote:
>
>> Here is a prototype patch of the compressed IRQ stubs -- this patch
>> compresses them down to 7 stubs per 32-byte cache line (or part of cache
>> line) at the expense of a back-to-back jmp which has the potential of
>> being ugly on some pipelines (we can only get 4 stubs into 32 bytes
>> without that).
>>   
>
> You could actually get 4-byte stubs, using a 16-bit call (66 e8 ww 
> ww).  But it would be slower, since we won't be pairing it with a ret.
>
> I suspect we could get it down to three bytes, by sharing the last 
> byte of the four-byte call sequence with the first byte of the next:
>
>  66 e8 ff 66 e8 fc 66 e8 f9 66 e8 f6 ...
>
> Every three bytes a new stub begins; it's a four-byte call to offset 
> 0x6703 relative to the beginning of the first stub.
>
> Can anyone better 24 bits/stub?

I actually got it down to 16 bits: use a 16-bit code segment, so you can 
drop the address size override:

e8 ff e8 fd e8 fb ...

of course the common code has to jump back to a 32-bit code segment.

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

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