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Date:   Thu, 14 Sep 2023 08:09:46 +0200
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Subject: Re: [PATCH v10 08/38] x86/cpufeatures: Add the cpu feature bit for
 FRED
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To:     Juergen Gross <jgross@suse.com>
Cc:     tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com,
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        peterz@infradead.org, ravi.v.shankar@intel.com,
        mhiramat@kernel.org, andrew.cooper3@citrix.com,
        jiangshanlai@gmail.com, Xin Li <xin3.li@intel.com>,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
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References: <20230914044805.301390-1-xin3.li@intel.com>
 <20230914044805.301390-9-xin3.li@intel.com>
 <d98a362d-d806-4458-9473-be5bea254db7@suse.com>
From:   Jan Beulich <jbeulich@suse.com>
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On 14.09.2023 08:03, Juergen Gross wrote:
> On 14.09.23 06:47, Xin Li wrote:
>> From: "H. Peter Anvin (Intel)" <hpa@zytor.com>
>>
>> Any FRED CPU will always have the following features as its baseline:
>>    1) LKGS, load attributes of the GS segment but the base address into
>>       the IA32_KERNEL_GS_BASE MSR instead of the GS segment’s descriptor
>>       cache.
>>    2) WRMSRNS, non-serializing WRMSR for faster MSR writes.
>>
>> Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
>> Tested-by: Shan Kang <shan.kang@intel.com>
>> Signed-off-by: Xin Li <xin3.li@intel.com>
> 
> In order to avoid having to add paravirt support for FRED I think
> xen_init_capabilities() should gain:
> 
> +    setup_clear_cpu_cap(X86_FEATURE_FRED);

I don't view it as very likely that Xen would expose FRED to PV guests
(Andrew?), at which point such a precaution may not be necessary.

Jan