Re: [Xen-devel] [PATCH 06/13] x86/hvm: Scale host TSC when setting/getting guest TSC

[Haozhong Zhang <haozhong.zhang@xxxxxxxxx>]

On Thu, Oct 22, 2015 at 08:17:29AM -0600, Jan Beulich wrote:
> >>> On 28.09.15 at 09:13, <haozhong.zhang@xxxxxxxxx> wrote:
> > The existing hvm_set_guest_tsc_fixed() and hvm_get_guest_tsc_fixed()
> > calculate the guest TSC by adding the TSC offset to the host TSC. When
> > the TSC scaling is enabled, the host TSC should be scaled first. This
> > patch adds the scaling logic to those two functions.
> 
> Just like mentioned for the first twp patches - I'd first of all like to
> understand why the lack of scaling this wasn't an issue for SVM so
> far. What you reads plausible, but assuming that SVM TSC scaling
> code was tested, I'm hesitant to apply changes to it without
> understanding the details (or at least without SVM maintainers'
> consent).
> 
> > --- a/xen/arch/x86/hvm/hvm.c
> > +++ b/xen/arch/x86/hvm/hvm.c
> > @@ -388,13 +388,12 @@ void hvm_set_guest_tsc_fixed(struct vcpu *v, u64 
> > guest_tsc, u64 at_tsc)
> >          tsc = hvm_get_guest_time_fixed(v, at_tsc);
> >          tsc = gtime_to_gtsc(v->domain, tsc);
> >      }
> > -    else if ( at_tsc )
> > -    {
> > -        tsc = at_tsc;
> > -    }
> >      else
> >      {
> > -        tsc = rdtsc();
> > +        tsc = at_tsc ? at_tsc : rdtsc();
> 
> In cases like this please prefer the gcc extension allowing the middle
> operand of the ?: to be omitted.
>

will modify

> Jan
> 

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