From: Andrew Cooper <andrew.cooper3@citrix.com>
To: "Roger Pau Monné" <roger.pau@citrix.com>
Cc: xen-devel@lists.xenproject.org, Wei Liu <wl@xen.org>,
Jan Beulich <jbeulich@suse.com>
Subject: Re: [Xen-devel] [PATCH] x86/hvm: always expose x2APIC feature in max HVM cpuid policy
Date: Tue, 24 Dec 2019 16:00:27 +0000 [thread overview]
Message-ID: <950e6538-40f4-c698-75d9-f532bccfae52@citrix.com> (raw)
In-Reply-To: <20191224124258.GC11756@Air-de-Roger>
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@citrix.com>
>> 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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next prev parent reply other threads:[~2019-12-24 16:01 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-24 10:18 [Xen-devel] [PATCH] x86/hvm: always expose x2APIC feature in max HVM cpuid policy Roger Pau Monne
2019-12-24 12:23 ` Andrew Cooper
2019-12-24 12:42 ` Roger Pau Monné
2019-12-24 16:00 ` Andrew Cooper [this message]
2019-12-24 18:17 ` Roger Pau Monné
2020-01-09 10:48 ` Roger Pau Monné
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