linux-kernel - Re: [PATCH v10 03/38] x86/msr: Add the WRMSRNS instruction support

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Date:   Thu, 14 Sep 2023 08:02:37 +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 03/38] x86/msr: Add the WRMSRNS instruction support

On 14.09.23 06:47, Xin Li wrote:
> Add an always inline API __wrmsrns() to embed the WRMSRNS instruction
> into the code.
> 
> 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 WRMSRNS I think
xen_init_capabilities() should gain:

+	setup_clear_cpu_cap(X86_FEATURE_WRMSRNS);


Juergen

> ---
>   arch/x86/include/asm/msr.h | 18 ++++++++++++++++++
>   1 file changed, 18 insertions(+)
> 
> diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
> index 65ec1965cd28..c284ff9ebe67 100644
> --- a/arch/x86/include/asm/msr.h
> +++ b/arch/x86/include/asm/msr.h
> @@ -97,6 +97,19 @@ static __always_inline void __wrmsr(unsigned int msr, u32 low, u32 high)
>   		     : : "c" (msr), "a"(low), "d" (high) : "memory");
>   }
>   
> +/*
> + * WRMSRNS behaves exactly like WRMSR with the only difference being
> + * that it is not a serializing instruction by default.
> + */
> +static __always_inline void __wrmsrns(u32 msr, u32 low, u32 high)
> +{
> +	/* Instruction opcode for WRMSRNS; supported in binutils >= 2.40. */
> +	asm volatile("1: .byte 0x0f,0x01,0xc6\n"
> +		     "2:\n"
> +		     _ASM_EXTABLE_TYPE(1b, 2b, EX_TYPE_WRMSR)
> +		     : : "c" (msr), "a"(low), "d" (high));
> +}
> +
>   #define native_rdmsr(msr, val1, val2)			\
>   do {							\
>   	u64 __val = __rdmsr((msr));			\
> @@ -297,6 +310,11 @@ do {							\
>   
>   #endif	/* !CONFIG_PARAVIRT_XXL */
>   
> +static __always_inline void wrmsrns(u32 msr, u64 val)
> +{
> +	__wrmsrns(msr, val, val >> 32);
> +}
> +
>   /*
>    * 64-bit version of wrmsr_safe():
>    */


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