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 08:03:30 +0200
From:   Juergen Gross <jgross@...e.com>
To:     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
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
Subject: Re: [PATCH v10 08/38] x86/cpufeatures: Add the cpu feature bit for
 FRED

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);


Juergen

> ---
>   arch/x86/include/asm/cpufeatures.h       | 1 +
>   arch/x86/kernel/cpu/cpuid-deps.c         | 2 ++
>   tools/arch/x86/include/asm/cpufeatures.h | 1 +
>   3 files changed, 4 insertions(+)
> 
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 330876d34b68..57ae93dc1e52 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -321,6 +321,7 @@
>   #define X86_FEATURE_FZRM		(12*32+10) /* "" Fast zero-length REP MOVSB */
>   #define X86_FEATURE_FSRS		(12*32+11) /* "" Fast short REP STOSB */
>   #define X86_FEATURE_FSRC		(12*32+12) /* "" Fast short REP {CMPSB,SCASB} */
> +#define X86_FEATURE_FRED		(12*32+17) /* Flexible Return and Event Delivery */
>   #define X86_FEATURE_LKGS		(12*32+18) /* "" Load "kernel" (userspace) GS */
>   #define X86_FEATURE_WRMSRNS		(12*32+19) /* "" Non-Serializing Write to Model Specific Register instruction */
>   #define X86_FEATURE_AMX_FP16		(12*32+21) /* "" AMX fp16 Support */
> diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c
> index e462c1d3800a..b7174209d855 100644
> --- a/arch/x86/kernel/cpu/cpuid-deps.c
> +++ b/arch/x86/kernel/cpu/cpuid-deps.c
> @@ -82,6 +82,8 @@ static const struct cpuid_dep cpuid_deps[] = {
>   	{ X86_FEATURE_XFD,			X86_FEATURE_XGETBV1   },
>   	{ X86_FEATURE_AMX_TILE,			X86_FEATURE_XFD       },
>   	{ X86_FEATURE_SHSTK,			X86_FEATURE_XSAVES    },
> +	{ X86_FEATURE_FRED,			X86_FEATURE_LKGS      },
> +	{ X86_FEATURE_FRED,			X86_FEATURE_WRMSRNS   },
>   	{}
>   };
>   
> diff --git a/tools/arch/x86/include/asm/cpufeatures.h b/tools/arch/x86/include/asm/cpufeatures.h
> index 1b9d86ba5bc2..18bab7987d7f 100644
> --- a/tools/arch/x86/include/asm/cpufeatures.h
> +++ b/tools/arch/x86/include/asm/cpufeatures.h
> @@ -317,6 +317,7 @@
>   #define X86_FEATURE_FZRM		(12*32+10) /* "" Fast zero-length REP MOVSB */
>   #define X86_FEATURE_FSRS		(12*32+11) /* "" Fast short REP STOSB */
>   #define X86_FEATURE_FSRC		(12*32+12) /* "" Fast short REP {CMPSB,SCASB} */
> +#define X86_FEATURE_FRED		(12*32+17) /* Flexible Return and Event Delivery */
>   #define X86_FEATURE_LKGS		(12*32+18) /* "" Load "kernel" (userspace) GS */
>   #define X86_FEATURE_WRMSRNS		(12*32+19) /* "" Non-Serializing Write to Model Specific Register instruction */
>   #define X86_FEATURE_AMX_FP16		(12*32+21) /* "" AMX fp16 Support */


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