Re: [Xen-devel] [PATCH] x86/cpuid: fix dom0 crash on skylake machine

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 01/06/16 13:01, Jan Beulich wrote:
>>
>>>> I want to adjust the representation of cpuid information in struct
>>>> domain. The current loop in domain_cpuid() causes an O(N) overhead for
>>>> every query, which is very poor for actions which really should be a
>>>> single bit test at a fixed offset.
>>>>
>>>> This needs to be combined with properly splitting the per-domain and
>>>> per-vcpu information, which requires knowing the expected vcpu topology
>>>> during domain creation.
>>>>
>>>> On top of that, there needs to be verification logic to check the
>>>> correctness of information passed from the toolstack.
>>>>
>>>> All of these areas are covered in the "known issues" section of the
>>>> feature doc, and I do plan to fix them all.  However, it isn't a couple
>>>> of hours worth of work.
>>> All understood, yet not to the point: The original remark was that
>>> the very XSTATE handling could be done better with far not as much
>>> of a change, at least afaict without having tried.
>> In which case I don't know what you were suggesting.
> Make {hvm,pv}_cpuid() invoke themselves recursively to
> determine what bits to mask off from CPUID[0xd].EAX.

So that would work.  However, to do this, you need to query leaves 1,
0x80000001 and 7, all of which will hit the O(N) loop in domain_cpuid()

Luckily, none of those specific paths further recurse into {hvm,pv}_cpuid().

I am unsure which to go with.  My gut feel is that this would be quite a
performance hit, but I have no evidence either way.  OTOH, it will give
the correct answer, rather than an approximation.

~Andrew

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