Re: [Xen-devel] Hypervisor cpuid time leaf

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 03.05.17 at 16:06, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 03/05/17 14:57, Boris Ostrovsky wrote:
>> Is there a reason why we don't document hypervisor time leaf (3, or is
>> it 4?) in public/arch-x86/cpuid.h?
> 
> (The leaf with the number 3.  The way the documentation refers to leaves
> and numeric values is very counter-intuitive.  I half remember a plan to
> renumber the comments to be zero-based, which then match the constants. 
> Also, XEN_CPUID_MAX_NUM_LEAVES is entirely erroneous to have in the
> public API.)

I agree.

>> We have a regression in Linux where there is a window when
>> vcpu_time_info data is not yet available and one possibility is to use
>> this leaf. But I'd like to be sure it is part of a stable ABI.
> 
> One problem it has is that there is no indication of the valid subleafs
> (a problem shared with the subsequent leaf).  I'd like to get agreement
> on how to sort that (possibly via documentation only) before declaring
> the ABI stable.

I don't think we can call this ABI unstable, considering for how long
we've been shipping releases with it. The lack of valid subleaf
indication is not very problematic imo as long as all invalid ones
properly return all zeros. Of course that doesn't mean I'm opposed
to (a) avoiding the issue in future leaves and (b) fixing the issue in
the HVM leaf (where we still can without breaking the ABI, just not
via the usual way of providing the maximum valid number in EAX).

Jan


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