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

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Message-Id: <1225824256.28612.1282966677@webmail.messagingengine.com>
Date:	Tue, 04 Nov 2008 19:44:16 +0100
From:	"Alexander van Heukelum" <heukelum@...tmail.fm>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	"Andi Kleen" <andi@...stfloor.org>,
	"Cyrill Gorcunov" <gorcunov@...il.com>,
	"Alexander van Heukelum" <heukelum@...lshack.com>,
	"LKML" <linux-kernel@...r.kernel.org>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Thomas Gleixner" <tglx@...utronix.de>, lguest@...abs.org,
	jeremy@...source.com, "Steven Rostedt" <srostedt@...hat.com>,
	"Mike Travis" <travis@....com>
Subject: Re: [PATCH RFC/RFB] x86_64, i386: interrupt dispatch changes


On Tue, 04 Nov 2008 10:14:11 -0800, "H. Peter Anvin" <hpa@...or.com>
said:
> Alexander van Heukelum wrote:
> > 
> > That's good to know. I assume this LOCKed bus cycle only occurs
> > if the (hidden) segment information is not cached in some way?
> > How many segments are typically cached? In particular, does it
> > optimize switching between two segments?
> > 
> 
> Yes, there is a segment descriptor cache (as opposed to the hidden but
> architectural segment descriptor *registers*, which the Intel
> documentation confusingly call a "cache".)
> 
> It is used to optimize switching between a small number of segments, and
> was crucial for decent performance on Win9x, which contained a bunch of
> 16-bit code.

Thanks for the info!

This just means that if there are performance problems, the
'specialized'
handlers should be using the kernel segment or maybe a single common
segment. It would still allow us to get rid of the trampolines. A stack
trace should be enough to reconstruct which vector was originally called
in that case. Only the common_interrupt-codepath needs the original
vector as far as I can see.

You just made testing on larger machines with a lot of external
interrupts necessary :-/. (Assuming small machines do not show
performance problems, that is.)

Greetings,
    Alexander

> 	-hpa
-- 
  Alexander van Heukelum
  heukelum@...tmail.fm

-- 
http://www.fastmail.fm - I mean, what is it about a decent email service?

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