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

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Date:	Tue, 04 Nov 2008 10:14:11 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Alexander van Heukelum <heukelum@...tmail.fm>
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

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.

	-hpa
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