Re: [PATCH v3 17/18] x86/kvm/tsc: Drop extra barrier and use rdtsc_ordered in kvmclock

[Paolo Bonzini]

On 17/06/2015 02:36, Andy Lutomirski wrote:
> __pvclock_read_cycles had an unnecessary barrier.  Get rid of that
> barrier and clean up the code by just using rdtsc_ordered().
> 
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim Krcmar <rkrcmar@xxxxxxxxxx>
> Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> Cc: kvm@xxxxxxxxxxxxxxx
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> ---
> 
> I'm hoping to get an ack for this to go in through -tip.  (Arguably
> I'm the maintainer of this code given how it's used, but I should
> still ask for an ack.)
> 
> arch/x86/include/asm/pvclock.h | 21 ++++++++++++---------
>  1 file changed, 12 insertions(+), 9 deletions(-)

Can you send a URL to the rest of the series?  I've never even seen v1
or v2 so I have no idea of what this is about.

> diff --git a/arch/x86/include/asm/pvclock.h b/arch/x86/include/asm/pvclock.h
> index 6084bce345fc..cf2329ca4812 100644
> --- a/arch/x86/include/asm/pvclock.h
> +++ b/arch/x86/include/asm/pvclock.h
> @@ -62,7 +62,18 @@ static inline u64 pvclock_scale_delta(u64 delta, u32 mul_frac, int shift)
>  static __always_inline
>  u64 pvclock_get_nsec_offset(const struct pvclock_vcpu_time_info *src)
>  {
> -	u64 delta = rdtsc() - src->tsc_timestamp;
> +	/*
> +	 * Note: emulated platforms which do not advertise SSE2 support
> +	 * break rdtsc_ordered, resulting in kvmclock not using the
> +	 * necessary RDTSC barriers.  Without barriers, it is possible
> +	 * that RDTSC instruction is executed before prior loads,
> +	 * resulting in violation of monotonicity.
> +	 *
> +	 * On an SMP guest without SSE2, it's unclear how anything is
> +	 * supposed to work correctly, though -- memory fences
> +	 * (e.g. smp_mb) are important for more than just timing.
> +	 */

On an SMP guest without SSE2, memory fences are obtained with e.g. "lock
addb $0, (%esp)".

> +	u64 delta = rdtsc_ordered() - src->tsc_timestamp;
>  	return pvclock_scale_delta(delta, src->tsc_to_system_mul,
>  				   src->tsc_shift);
>  }
> @@ -76,17 +87,9 @@ unsigned __pvclock_read_cycles(const struct pvclock_vcpu_time_info *src,
>  	u8 ret_flags;
>  
>  	version = src->version;
> -	/* Note: emulated platforms which do not advertise SSE2 support
> -	 * result in kvmclock not using the necessary RDTSC barriers.
> -	 * Without barriers, it is possible that RDTSC instruction reads from
> -	 * the time stamp counter outside rdtsc_barrier protected section
> -	 * below, resulting in violation of monotonicity.
> -	 */
> -	rdtsc_barrier();
>  	offset = pvclock_get_nsec_offset(src);
>  	ret = src->system_time + offset;
>  	ret_flags = src->flags;
> -	rdtsc_barrier();
>  
>  	*cycles = ret;
>  	*flags = ret_flags;
> 
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