Re: [Xen-devel] [PATCH RFC 0/8] x86/hvm, libxl: HVM SMT topology support

[Joao Martins <joao.m.martins@xxxxxxxxxx>]

On 03/03/2016 10:24 AM, Andrew Cooper wrote:
> On 03/03/16 09:52, Joao Martins wrote:
>>
>>>>> In particular, I am concerned about giving the toolstack the ability to
>>>>> blindly control the APIC IDs.  Their layout is very closely linked to
>>>>> topology, and in particular to the HTT flag.
>>>>>
>>>>> Overall, I want to avoid any possibility of generating APIC layouts
>>>>> (including the emulated IOAPIC with HVM guests) which don't conform to
>>>>> the appropriate AMD/Intel manuals.
>>>> I see so overall having Xen control the topology would be a better 
>>>> approach that
>>>> "mangling" the APICIDs in the cpuid policy as I am proposing. One good 
>>>> thing
>>>> about Xen handling the topology bits would be for Intel CPUs with CPUID 
>>>> faulting
>>>> support where PV guests could also see the topology info. And given that 
>>>> the
>>>> word 10 of hw_caps won't be exposed (as per your CPUID), handling the PV 
>>>> case on
>>>> cpuid policy wouldn't be as clean.
>>> Which word do you mean here?  Even before my series, Xen only had 9
>>> words in hw_cap.
>> Hm, I used the wrong nomenclature here: what I meant was the 10th feature 
>> word
>> from x86_boot_capability (since the sysctl/libxl are capped to 8 words only)
>> which in the header files is word 9 on your series (previously moved from 
>> word
>> 3). It's the one meant for "Other features, Linux-defined mapping", where
>> X86_FEATURE_CPUID_FAULTING is defined.
> 
> Ah - so the word of synthetic values.
> 
> I don't see how the lack of that word makes policy handling any harder? 
> All information in there is gathered from other sources.
I meant specifically with the approach of my series i.e. changing cpuid policy
for HVM guests. PV guests cpuid is only trapped with CPUID_FAULTING so adding
support for PV guests topology on this toolstack-based approach we would need to
perhaps expose CPUID_FAULTING support somehow in physinfo so that the toolstack
know when to safely set the guest topology.

Joao

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