linux-kernel - Re: [PATCH v10 08/38] x86/cpufeatures: Add the cpu feature bit for FRED

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Date:   Thu, 14 Sep 2023 08:09:46 +0200
From:   Jan Beulich <jbeulich@...e.com>
To:     Juergen Gross <jgross@...e.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        luto@...nel.org, pbonzini@...hat.com, seanjc@...gle.com,
        peterz@...radead.org, ravi.v.shankar@...el.com,
        mhiramat@...nel.org, andrew.cooper3@...rix.com,
        jiangshanlai@...il.com, Xin Li <xin3.li@...el.com>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-edac@...r.kernel.org, linux-hyperv@...r.kernel.org,
        kvm@...r.kernel.org, xen-devel@...ts.xenproject.org
Subject: Re: [PATCH v10 08/38] x86/cpufeatures: Add the cpu feature bit for
 FRED

On 14.09.2023 08:03, Juergen Gross wrote:
> On 14.09.23 06:47, Xin Li wrote:
>> From: "H. Peter Anvin (Intel)" <hpa@...or.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@...or.com>
>> Tested-by: Shan Kang <shan.kang@...el.com>
>> Signed-off-by: Xin Li <xin3.li@...el.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