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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan


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