Re: [Xen-devel] [PATCH RFC 02/31] tools/libxc: Use public/featureset.h for cpuid policy generation

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 05/01/16 15:02, Ian Campbell wrote:
> On Tue, 2016-01-05 at 14:23 +0000, Andrew Cooper wrote:
>> On 05/01/16 14:18, Ian Campbell wrote:
>>> On Tue, 2016-01-05 at 14:17 +0000, Andrew Cooper wrote:
>>>> On 05/01/16 14:13, Ian Campbell wrote:
>>>>> On Tue, 2015-12-22 at 09:29 -0700, Jan Beulich wrote:
>>>>>>>>> On 16.12.15 at 22:24, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>>>>> --- a/xen/include/public/arch-x86/featureset.h
>>>>>>> +++ b/xen/include/public/arch-x86/featureset.h
>>>>>>> @@ -163,6 +163,7 @@
>>>>>>>  
>>>>>>>  /* Intel-defined CPU features, CPUID level 0x00000007:0.ebx,
>>>>>>> word
>>>>>>> 5 */
>>>>>>>  #define X86_FEATURE_FSGSBASE      ( 5*32+ 0) /*
>>>>>>> {RD,WR}{FS,GS}BASE
>>>>>>> instructions */
>>>>>>> +#define X86_FEATURE_TSC_ADJUST    ( 5*32+ 1) /* TSC_ADJUST MSR
>>>>>>> available */
>>>>>> This would probably better go into patch 1.
>>>>> Tools would then see this defined twice with only patch 1 applied,
>>>>> and
>>>>> since the value is actually different I think the compiler will
>>>>> complain.
>>>>>
>>>>> -#define X86_FEATURE_TSC_ADJUST   1 /* Tsc thread offset */
>>>>> +#define X86_FEATURE_TSC_ADJUST    ( 5*32+ 1) /* TSC_ADJUST MSR
>>>>> available */
>>>>>
>>>>> The comment change seems to be a semantic one? Or was it wrong
>>>>> beofre?
>>>> Changing patch 1 won't affect the compilation of libxc.  Observe in
>>>> the
>>>> penultimate hunk that I also change the #include
>>> Ah yes.
>>>
>>> What about my comment on the comment changing?
>> Ah yes - the changes are just semantic.  Also observe that the same hunk
>> also modifies the libxc macros to modulo 32.
> I'm not talking about the value at all, since I realised that the masking
> makes the values the same. I'm talking about the /* comment */ next to it
> which has also changed.
>
>> In the end, both the hypervisor and libxc are dealing with
>> hardware-specified bits in registers.  This patch is no resulting change
>> to behaviour.
> But do "Tsc thread offset" and "TSC_ADJUST MSR available" mean the same
> thing? Or was "Tsc thread offset" previously actively wrong or misleading
> in some way?

The latter is the accurate meaning of the bit in cpuid.  The former is
misleading IMO, and covers one use for it in a Linux world.

~Andrew

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