Re: [Xen-devel] [PATCH v5 1/6] x86/hvm: Collect information of TSC scaling ratio

[Haozhong Zhang <haozhong.zhang@xxxxxxxxx>]

On 02/23/16 08:37, Jan Beulich wrote:
> >>> On 23.02.16 at 15:16, <boris.ostrovsky@xxxxxxxxxx> wrote:
> > On 02/23/2016 09:10 AM, Jan Beulich wrote:
> >>>>> On 23.02.16 at 15:00, <boris.ostrovsky@xxxxxxxxxx> wrote:
> >>> On 02/22/2016 09:04 PM, Haozhong Zhang wrote:
> >>>
> >>>>    
> >>>> +    if ( cpu_has_tsc_ratio )
> >>>> +        svm_function_table.tsc_scaling.ratio_frac_bits = 32;
> >>>> +
> >>>
> >>>>    
> >>>> +#define hvm_tsc_scaling_supported \
> >>>> +    (!!hvm_funcs.tsc_scaling.ratio_frac_bits)
> >>>> +
> >>> What is the difference (in usage) between cpu_has_tsc_ratio and
> >>> hvm_tsc_scaling_supported? Isn't the first imply the second (and if yes
> >>> then what's the reason for having the latter)?
> >> Iiuc cpu_has_tsc_ratio is AMD/SVM specific, while
> >> hvm_tsc_scaling_supported is meant to be vendor independent.
> >

Yes, it's to be vendor independent. Earlier versions of this patch
series set a field tsc_scaling_supported in hvm_function_table if
cpu_has_vmx_tsc_scaling or cpu_has_tsc_ratio. Jan suggested we could get
the same information if some of other fields are initialized
conditionally, and no extra field (tsc_scaling_supported) would be
needed any more.

> > Ah, OK. Can we then
> > 
> > #define hvm_tsc_scaling_supported (cpu_has_vmx_tsc_scaling || 
> > cpu_has_tsc_ratio)
> 
> Why would we? The above is doing precisely (but implicitly) that,
> just with only one memory access instead of two.
>

Boris, does the current one look fine for you, as it does the same thing
as your suggested one?

Thanks,
Haozhong

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