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

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Date:	Fri, 09 Mar 2007 20:10:03 +0200
From:	Avi Kivity <avi@...o.co.il>
To:	Joerg Roedel <joerg.roedel@....com>
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()

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?


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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