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Re: [Xen-devel] [PATCH] x86/hvm: always expose x2APIC feature in max HVM cpuid policy
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Tue, 24 Dec 2019 16:00:27 +0000
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- Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx, Wei Liu <wl@xxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>
- Delivery-date: Tue, 24 Dec 2019 16:00:50 +0000
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On 24/12/2019 12:42, Roger Pau Monné wrote:
> On Tue, Dec 24, 2019 at 12:23:12PM +0000, Andrew Cooper wrote:
>> On 24/12/2019 10:18, Roger Pau Monne wrote:
>>> On hardware without x2APIC support Xen emulated local APIC will
>>> provide such mode, and hence the feature should be set in the maximum
>>> HVM cpuid policy.
>>>
>>> Not exposing it in the maximum policy results in HVM domains not
>>> getting such feature exposed unless it's also supported by the
>>> underlying hardware.
>>>
>>> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>> x2apic has never been exposed via this mechanism, due to its effects on
>> topology calculations.
> Well, it's exposed in hvm_max_cpuid_policy if it's present in the
> hardware. IMO it's kind of weird that the presence of the x2APIC feature
> on the max policy depends on the underlying hardware, when it's always
> supported by the emulated vlapic.
>
> I think x2APIC must either always be part of the max policy, or never,
> or else it's very easy to cause regressions because it's not so easy
> to find a box without x2APIC.
Hmm - this does highlight the inconsistency in the existing logic. I'm
not overly surprised - this is a remnant of the old model which hasn't
been rewritten yet.
>
>> It has however always been down to the toolstack to opt in, and Xen
>> permits this via recalculate_cpuid_policy(), on the expectation that the
>> toolstack knew what it was doing WRT topology (all evidence actually to
>> the contrary).
> Hm, I can try to force the setting in libxc.
>
>> If we're seeing a recent change in behaviour, then it will be from libxc.
> IIRC x2APIC was always exposed to HVM guests regardless of the
> underlying hardware support.
I have a suspicion that this may have been broken by c/s 3e0c8272f in
2015...
~Andrew
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