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Re: [PATCH] libx86: Introduce x86_cpu_policy_calculate_compatible() with MSR_ARCH_CAPS handling
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 5 May 2021 16:18:10 +0100
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- Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Wed, 05 May 2021 15:18:27 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 05/05/2021 16:00, Jan Beulich wrote:
> On 05.05.2021 16:50, Andrew Cooper wrote:
>> On 05/05/2021 15:48, Jan Beulich wrote:
>>> On 05.05.2021 16:29, Andrew Cooper wrote:
>>>> Technically, MCXSR_MASK is also a hard blocker to migration, but we
>>>> don't even have that data in a consumable form, and we just might be
>>>> extremely lucky and discover that it is restricted to non-64-bit CPUs.
>>> "it" being what here? The value's presence / absence in an {F,}XSAVE
>>> image? Or the precise value of it?
>> The precise value of it.
> Not sure whether DAZ is new enough, but relatively sure MM isn't.
>
>> Migrating across the boundary where the
>> default changed will cause {F,}RSTOR instructions to #GP.
> That's understood. Is there actually anything standing in the way
> of treating MXCSR_MASK like yet another feature flag group?
Well - we'd need to find somewhere to put it. It doesn't fit in
architectural CPUID or MSR information.
We could add a 3rd category of information in "a cpu policy", and that
is always an option.
However, this issue hasn't been a problem so far, is only for very
legacy CPUs at this point, and isn't high on my list of worries.
~Andrew
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