Message-ID: <492DC49C.7050101@redhat.com>
Date: Wed, 26 Nov 2008 23:50:20 +0200
From: Avi Kivity <avi@redhat.com>
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To: "H. Peter Anvin" <hpa@zytor.com>
CC: Alexander van Heukelum <heukelum@fastmail.fm>, lguest@ozlabs.org,
       jeremy@xensource.com, LKML <linux-kernel@vger.kernel.org>,
       Mike Travis <travis@sgi.com>, Cyrill Gorcunov <gorcunov@gmail.com>,
       Steven Rostedt <srostedt@redhat.com>, Andi Kleen <andi@firstfloor.org>,
       Ingo Molnar <mingo@elte.hu>,
       Alexander van Heukelum <heukelum@mailshack.com>,
       Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [Lguest] [PATCH RFC/RFB] x86_64,	i386: interrupt dispatch changes
References: <20081104122839.GA22864@mailshack.com>	<20081104150729.GC21470@localhost>	<20081104170501.GE29626@one.firstfloor.org>	<1225822006.21441.1282961299@webmail.messagingengine.com>	<20081104204400.GC10825@elte.hu>	<1226243805.27361.1283784629@webmail.messagingengine.com> <49178E89.2000307@zytor.com> <492DC13F.8020009@redhat.com>
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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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