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

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Message-ID: <20070312132155.GF8922@amd.com>
Date:	Mon, 12 Mar 2007 14:21:55 +0100
From:	"Joerg Roedel" <joerg.roedel@....com>
To:	"Andi Kleen" <ak@...e.de>
cc:	discuss@...-64.org, "Avi Kivity" <avi@...o.co.il>,
	linux-kernel@...r.kernel.org
Subject: Re: [discuss] [PATCH 4/4 TRY#3] optimize and simplify
 get_cycles_sync()

On Mon, Mar 12, 2007 at 02:09:18PM +0100, Andi Kleen wrote:
> On Monday 12 March 2007 14:02, Joerg Roedel wrote:
> > 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.
> 
> It doesn't exist. The rdtscp macros are also not used currently, that
> is why nobody's binutils complained.
> 
> Doing the .bytes is ok
> 
> I still don't like the alternative() record complications though.

Do you think of another way to make use of RDTSCP in the get_cycles_sync
function? Using CPUID in a function called such often is bad when
running Linux as a  virtualization guest...
So using RDTSCP there might be a goog idea.

Regards,
Joerg

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


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