From mboxrd@z Thu Jan  1 00:00:00 1970
From: Haozhong Zhang <haozhong.zhang@intel.com>
Subject: Re: [PATCH v5 1/6] x86/hvm: Collect information of TSC
	scaling ratio
Date: Wed, 24 Feb 2016 14:00:20 +0800
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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>
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On 02/23/16 08:37, Jan Beulich wrote:
> >>> On 23.02.16 at 15:16, <boris.ostrovsky@oracle.com> wrote:
> > On 02/23/2016 09:10 AM, Jan Beulich wrote:
> >>>>> On 23.02.16 at 15:00, <boris.ostrovsky@oracle.com> 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