linux-kernel - Re: [PATCH 4/4 TRY#3] optimize and simplify get_cycles

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Message-ID: <20070312130245.GE8922@amd.com>
Date:	Mon, 12 Mar 2007 14:02:45 +0100
From:	"Joerg Roedel" <joerg.roedel@....com>
To:	"Avi Kivity" <avi@...o.co.il>
cc:	discuss@...-64.org, "Andi Kleen" <ak@...e.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4 TRY#3] optimize and simplify get_cycles_sync()

On Fri, Mar 09, 2007 at 08:10:03PM +0200, Avi Kivity wrote:
> Joerg Roedel wrote:
> >From: Joerg Roedel <joerg.roedel@....com>
> >
> >This patch simplifies the get_cycles_sync() function by removing
> >the #ifdefs from it. Further it introduces an optimization for AMD
> >processors. There the RDTSCP instruction is used instead of CPUID;RDTSC
> >which is helpfull if the kernel runs as a KVM guest. Running as a guest
> >makes CPUID very expensive because it causes an intercept of the guest.
> >
> >  +#define RDTSCP ".byte 0x0f, 0x01, 0xf9"
> >+	alternative_io_two("cpuid\nrdtsc",
> >+			   "rdtsc", X86_FEATURE_SYNC_RDTSC,
> >+			   ".byte 0x0f, 0x01, 0xf9", X86_FEATURE_RDTSCP,
> >  
> 
> why not use the RDTSCP macro here?

Does this macro exist? I couldn't found it in the current git tree. And
the rdtscp macros in msr.h use the plain opcode too.

Regards,
Joerg

-- 
Joerg Roedel
Operating System Research Center
AMD Saxony LLC & Co. KG


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