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Re: [Xen-devel] [PATCH v3 09/28] xen/x86: Calculate maximum host and guest featuresets

On 18/03/16 17:09, Jan Beulich wrote:
>>>> On 15.03.16 at 16:35, <andrew.cooper3@xxxxxxxxxx> wrote:
>> +static void __init calculate_hvm_featureset(void)
>> +{
>> +    unsigned int i;
>> +    const uint32_t *hvm_featuremask;
>> +
>> +    if ( !hvm_enabled )
>> +        return;
>> +
>> +    hvm_featuremask = hvm_funcs.hap_supported ?
>> +        hvm_hap_featuremask : hvm_shadow_featuremask;
> I know I asked about this before, and it still puzzles me. Could you
> add some explanation of this to the commit message or a comment?

I am not sure what more I can say about it.

The toolstack needs the be able to see the difference between a guest
started in shadow mode on hap hardware, to be able to correctly
calculate whether it can migrate to hap-incapable hardware. 
(Specifically - this is a usecase that I must support.)

There is no "get_cpuid_policy" hypercall, so a toolstack cannot query
what a guest can actually see, after Xen's dynamic checks have taken place.

Implementing get_cpuid_policy depends on Xen having a full model of
cpuid state, which is too much work to be rolled together into this series.

Like all of the dynamic checks later, it is only an intermediate step,
and I do have plans to remove them longterm when Xen has a better model
of cpuid.


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