linux-kernel - Re: lmbench lat_mmap slowdown with CONFIG

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Message-ID: <49790BCC.1040807@goop.org>
Date:	Thu, 22 Jan 2009 16:14:04 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Zachary Amsden <zach@...are.com>
CC:	Nick Piggin <npiggin@...e.de>, Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"hpa@...or.com" <hpa@...or.com>,
	"jeremy@...source.com" <jeremy@...source.com>,
	"chrisw@...s-sol.org" <chrisw@...s-sol.org>,
	"rusty@...tcorp.com.au" <rusty@...tcorp.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Xen-devel <xen-devel@...ts.xensource.com>
Subject: Re: lmbench lat_mmap slowdown with CONFIG_PARAVIRT

Zachary Amsden wrote:
> What about removing the identity functions entirely.  They are useless,
> really.  All that is needed is a patch site filled with nops for Xen to
> overwrite, just stuffing the value into the proper registers.  For
> 64-bit, it can be a simple mov to satisfy the constraints.
>   

I think it comes to the same thing really.  Both end up generating a 
series of nops with values entering and leaving in well-defined 
registers.  The x86-64 calling convention is a bit awkward because the 
first arg is in rdi and the ret is rax, so it can't quite be pure nops, 
or we use a non-standard calling-convention with appropriate thunks to 
call into C code.  I think a mov is a better performance-complexity 
tradeoff.

>> Also, I just posted patches to get rid of all pvops calls when fetching 
>> or setting flags in a pte, which I hope will help.
>>     
>
> Sounds like it will help.
>   

...but apparently not.

    J
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