Re: [Xen-devel] [PATCH RFC 2/2] xen/x86: Introduce XEN_SYSCTL_cpuid hypercall

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 27/02/14 12:26, Jan Beulich wrote:
>>>> On 27.02.14 at 13:11, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>> Apart from forcibly messing with a balanced numa setup, what about cpu
>> pools, or toolstack disaggregation where pinning is restricted?
> There no messing with a balanced setup here - everything should
> of course be transient, i.e. get restored to previous values right
> after.

But anything else happening on that vcpu at the same time is transiently
out of balance.

>  And I can't see a reason not to permit the hardware domain
> to temporarily escape its enclave, as it can do worse to the overall
> system anyway.

True

>
> The new hypercall is simple enough (yet very x86-centric) that I'm
> not really against it; what I'm against is adding functionality to the
> hypervisor that is already available by other means.
>
> Jan
>

As I said before, the cache information is faked up by the cpuid policy,
so might be subject to policy depending on faulting or non-dom0 hardware
domian, or PVH dom0 in the near future. 

I think there is a legitimate case for "I really truely need the real
hardware values, without anything in the policy getting in the way". 
'cpuid' is indeed very x86 specific, but it is not information readily
available by other means.

~Andrew

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