linux-kernel - Re: [PATCH v3 17/18] x86/kvm/tsc: Drop extra barrier and use rdtsc

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Date:	Sat, 20 Jun 2015 23:50:46 +0200
From:	Borislav Petkov <bp@...e.de>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
	Peter Zijlstra <peterz@...radead.org>,
	John Stultz <john.stultz@...aro.org>,
	linux-kernel@...r.kernel.org, Len Brown <lenb@...nel.org>,
	Huang Rui <ray.huang@....com>,
	Denys Vlasenko <dvlasenk@...hat.com>, kvm@...r.kernel.org,
	Ralf Baechle <ralf@...ux-mips.org>,
	Radim Krcmar <rkrcmar@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [PATCH v3 17/18] x86/kvm/tsc: Drop extra barrier and use
 rdtsc_ordered in kvmclock

On Wed, Jun 17, 2015 at 09:47:08AM +0200, Paolo Bonzini wrote:
> 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@...hat.com>
> > Cc: Radim Krcmar <rkrcmar@...hat.com>
> > Cc: Marcelo Tosatti <mtosatti@...hat.com>
> > Cc: kvm@...r.kernel.org
> > Signed-off-by: Andy Lutomirski <luto@...nel.org>
> > ---
> > 
> > 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)".

Yeah, I killed that comment when applying.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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