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Re: [PATCH 1/3] x86/amd: Use setup_force_cpu_cap() for BTC_NO
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Wed, 26 Nov 2025 15:12:36 +0000
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- Cc: andrew.cooper3@xxxxxxxxxx, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Wed, 26 Nov 2025 15:12:47 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 26/11/2025 2:19 pm, Jan Beulich wrote:
> On 26.11.2025 14:22, Andrew Cooper wrote:
>> When re-scanning features,
> What exactly do you mean with this, outside of XenServer (i.e. upstream)? The
> only thing I can think of is recheck_cpu_features(), which calls
> identify_cpu()
> and hence init_amd(). Thus ...
>
>> forced caps are taken into account but unforced
>> such as this are not. This causes BTC_NO to go missing, and for the system
>> to
>> appear to have lost features.
> ... I don't really follow where features might be lost.
Well - it's a feature that we started upstreaming and I still hope to
finish in some copious free time.
Already upstream, we rescan the Raw CPU policy after microcode load.
That has had fixes such as dis-engaging CPUID Masking/Overriding so the
Raw policy comes out accurate.
The next step (not upstream yet) is to regenerate the Host and Guest
policies. I recently fixed a bug in XenServer's testing and noticed
that the underlying logic had bit-rotted quite a bit, hence this series.
The purpose is to be able to activate new features added by a late
microcode load, such as new speculative defences, or simply provide new
FOO_NO bits.
~Andrew
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