From mboxrd@z Thu Jan  1 00:00:00 1970
From: Gleb Natapov <gleb@redhat.com>
Subject: Re: [patch 02/16] x86: pvclock: make sure rdtsc doesnt speculate out
 of region
Date: Thu, 1 Nov 2012 13:48:07 +0200
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 <20121031224823.907464421@redhat.com>
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To: Marcelo Tosatti <mtosatti@redhat.com>
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On Wed, Oct 31, 2012 at 08:46:58PM -0200, Marcelo Tosatti wrote:
> Originally from Jeremy Fitzhardinge.
> 
> pvclock_get_time_values, which contains the memory barriers
> will be removed by next patch.
> 
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> 
> Index: vsyscall/arch/x86/kernel/pvclock.c
> ===================================================================
> --- vsyscall.orig/arch/x86/kernel/pvclock.c
> +++ vsyscall/arch/x86/kernel/pvclock.c
> @@ -97,10 +97,10 @@ cycle_t pvclock_clocksource_read(struct 
>  
>  	do {
>  		version = pvclock_get_time_values(&shadow, src);
> -		barrier();
> +		rdtsc_barrier();
>  		offset = pvclock_get_nsec_offset(&shadow);
>  		ret = shadow.system_timestamp + offset;
> -		barrier();
> +		rdtsc_barrier();
>  	} while (version != src->version);
>  
>  	if ((valid_flags & PVCLOCK_TSC_STABLE_BIT) &&
> 
On a guest without SSE2 rdtsc_barrier() will be nop while rmb() will
be "lock; addl $0,0(%%esp)". I doubt pvclock will work correctly either
way though.

--
			Gleb.