Re: [PATCH v3 04/13] x86/time.c: Scale host TSC in pvclock properly

From: Haozhong Zhang <haozhong.zhang@intel.com>
To: Jan Beulich <JBeulich@suse.com>
Cc: Kevin Tian <kevin.tian@intel.com>, Keir Fraser <keir@xen.org>,
	Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	xen-devel@lists.xen.org,
	Aravind Gopalakrishnan <Aravind.Gopalakrishnan@amd.com>,
	Jun Nakajima <jun.nakajima@intel.com>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>
Subject: Re: [PATCH v3 04/13] x86/time.c: Scale host TSC in pvclock properly
Date: Fri, 8 Jan 2016 21:50:43 +0800	[thread overview]
Message-ID: <20160108135043.GE16393@hz-desktop.sh.intel.com> (raw)
In-Reply-To: <568FCB2002000078000C4E4A@prv-mh.provo.novell.com>

On 01/08/16 06:43, Jan Beulich wrote:
> >>> On 08.01.16 at 14:22, <haozhong.zhang@intel.com> wrote:
> > On 01/08/16 02:20, Jan Beulich wrote:
> >> >>> On 31.12.15 at 04:03, <haozhong.zhang@intel.com> wrote:
> >> > --- a/xen/arch/x86/time.c
> >> > +++ b/xen/arch/x86/time.c
> >> > @@ -815,10 +815,18 @@ static void __update_vcpu_system_time(struct vcpu *v, int force)
> >> >      }
> >> >      else
> >> >      {
> >> > -        tsc_stamp = t->local_tsc_stamp;
> >> > -
> >> > -        _u.tsc_to_system_mul = t->tsc_scale.mul_frac;
> >> > -        _u.tsc_shift         = (s8)t->tsc_scale.shift;
> >> > +        if ( has_hvm_container_domain(d) && cpu_has_tsc_ratio )
> >> 
> >> For symmetry with patches 1 and 2 I'd expect !d->arch.vtsc to be
> >> added here too.
> > 
> > even though it's already in the !d->arch.vtsc branch? But I'm fine to
> > add it for symmetric.
> 
> Ah, no, I didn't realize (only looking at the patch) that it's in the
> else to a respective if(). No need to change it then.
> 
> >> > +        {
> >> > +            tsc_stamp            = hvm_funcs.scale_tsc(v, 
> > t->local_tsc_stamp);
> >> > +            _u.tsc_to_system_mul = d->arch.vtsc_to_ns.mul_frac;
> >> > +            _u.tsc_shift         = d->arch.vtsc_to_ns.shift;
> >> > +        }
> >> > +        else
> >> > +        {
> >> > +            tsc_stamp            = t->local_tsc_stamp;
> >> > +            _u.tsc_to_system_mul = t->tsc_scale.mul_frac;
> >> > +            _u.tsc_shift         = (s8)t->tsc_scale.shift;
> >> 
> >> The case has been pointless anyway, and you don't add a similar
> >> one in the if() branch - please delete it.
> > 
> > I don't understand why it's pointless. The inner if branch is to
> > ensure pvclock provides the scaled TSC information when TSC scaling is
> > used, and the inner else branch is to keep the existing behavior when
> > TSC scaling is not used. For the outer if branch, TSC scaling is never
> > used, so no change is needed.
> 
> Sorry, I see I typo-ed my reply. It was meant to start "The cast
> has been ..."
> 
OK, I'll remove it.

Haozhong