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Re: [PATCH v6 1/7] x86: suppress ERMS for internal use when MISC_ENABLE.FAST_STRING is clear
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Sun, 29 Jun 2025 17:05:24 +0200
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Sun, 29 Jun 2025 15:05:40 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 26.06.2025 17:45, Jan Beulich wrote:
> On 26.06.2025 17:31, Andrew Cooper wrote:
>> On 16/06/2025 1:59 pm, Jan Beulich wrote:
>>> + if ( !raw_cpu_policy.feat.erms )
>>
>> This wants to be the host policy, not the raw policy. That's why
>> `cpuid=no-erms` isn't working in the way you'd expect.
>>
>> cpuid=no-$foo means "Xen should behave as if $foo wasn't reported by
>> hardware", and that includes not giving it to guests by default.
>
> Hmm, interesting. That's definitely not the meaning I give this. To me it
> means merely Xen shouldn't use the feature (with an impact on guests only
> when the feature in hardware is required to surface it to guests). I
> don't think we have the precise meaning of this option written down
> anywhere?
Then again I notice what you ask for is in line with uses of cpu_has_*
(where available) elsewhere (e.g. in your "x86/cpu-policy: Simplify logic
in guest_common_default_feature_adjustments()"), with
calculate_host_policy() simply copying boot_cpu_data.x86_capability into
the host policy. So yes, I guess I'll need to adjust.
Ftaod, this still leaves open what exact meaning "cpuid=no-..." ought to
have. While kind of an example in the opposite direction, consider e.g.
"cpuid=no-lm": This cannot sensibly have any effect on Xen itself. It
could plausibly mean to permit only 32-bit guests.
Jan
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