linux-kernel - Re: 2.6.{26.2,27-rc} oops on virtualbox

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Message-ID: <48B45F16.7080203@zytor.com>
Date:	Tue, 26 Aug 2008 12:52:54 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
CC:	"Luiz Fernando N. Capitulino" <lcapitulino@...driva.com.br>,
	Gerhard Brauer <gerhard.brauer@....de>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: 2.6.{26.2,27-rc} oops on virtualbox

One thing that I think really needs to be considered is that the current 
PV stubs are (a) large, and (b) non-atomic.

In the case at hand we have:

c012fc69:       51                      push   %ecx
c012fc6a:       52                      push   %edx
c012fc6b:       ff 15 40 b9 41 c0       call   *0xc041b940
c012fc71:       5a                      pop    %edx
c012fc72:       59                      pop    %ecx

Ten bytes replacing a two-byte native sequence.

If this was done as a call to an out-of-line stub, it would be only five 
bytes, which would reduce native icache overhead from 400% to 150%, but 
perhaps more importantly, it would not be subject to returns inside the 
sequence itself (since the out-of-line stub would still exist.)  As an 
optional bonus, at least on 32 bits the indirect call could be replaced 
with a direct call in the out-of-line stub.

	-hpa
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