Re: [Xen-devel] [RFC PATCH 1/8] x86/domctl: introduce a pair of hypercall to set and get cpu topology

[Chao Gao <chao.gao@xxxxxxxxx>]

On Tue, Jan 09, 2018 at 11:47:54PM +0000, Andrew Cooper wrote:
>On 08/01/18 04:01, Chao Gao wrote:
>> Define interface, structures and hypercalls for toolstack to build
>> cpu topology and for guest that will retrieve it [1].
>> Two subop hypercalls introduced by this patch:
>> XEN_DOMCTL_set_cpu_topology to define cpu topology information per domain
>> and XENMEM_get_cpu_topology to retrieve cpu topology information.
>>
>> [1]: during guest creation, those information helps hvmloader to build ACPI.
>>
>> Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
>
>I'm sorry, but this going in the wrong direction.  Details like this
>should be contained and communicated exclusively in the CPUID policy.
>
>Before the spectre/meltdown fire started, I had a prototype series
>introducing a toolstack interface for getting and setting a full CPUID
>policy at once, rather than piecewise.  I will be continuing with this

Is the new interface able to set CPUID policy for each vCPU rather than
current for each domain? Otherwise I couldn't see how to set APIC_ID
for each vcpu except by introducing a new interface.

>work once the dust settles.
>
>In particular, we should not have multiple ways of conveying the same
>information, or duplication of the same data inside the hypervisor.
>
>If you rearrange your series to put the struct cpuid_policy changes
>first, then patch 2 will become far more simple.  HVMLoader should
>derive its topology information from the CPUID instruction, just as is
>expected on native hardware.

Good point. It seems that in HVMLoader BSP should boot APs in a
broadcase fashion and then information is collected via CPUID and then
build MADT/SRAT.

Thanks
Chao

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