linux-kernel - Re: [PATCH 1/2] x86/msr: Rename the WRMSRNS opcode macro to ASM

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Message-ID: <ab5ded74-91b4-46ae-b360-b372ff790fa6@zytor.com>
Date: Wed, 26 Feb 2025 17:14:21 -0800
From: Xin Li <xin@...or.com>
To: Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] x86/msr: Rename the WRMSRNS opcode macro to
 ASM_WRMSRNS (for KVM)

On 2/26/2025 5:01 PM, Sean Christopherson wrote:
> Rename the WRMSRNS instruction opcode macro so that it doesn't collide
> with X86_FEATURE_WRMSRNS when using token pasting to generate references
> to X86_FEATURE_WRMSRNS.  KVM heavily uses token pasting to generate KVM's
> set of support feature bits, and adding WRMSRNS support in KVM will run
> will run afoul of the opcode macro.
> 
>    arch/x86/kvm/cpuid.c:719:37: error: pasting "X86_FEATURE_" and "" "" does not
>                                        give a valid preprocessing token
>    719 |         u32 __leaf = __feature_leaf(X86_FEATURE_##name);                \
>        |                                     ^~~~~~~~~~~~
> 
> KVM has worked around one such collision in the past by #undef'ing the
> problematic macro in order to avoid blocking a KVM rework, but such games
> are generally undesirable, e.g. requires bleeding macro details into KVM,
> risks weird behavior if what KVM is #undef'ing changes, etc.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/x86/include/asm/msr.h | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
> index 001853541f1e..60b80a36d045 100644
> --- a/arch/x86/include/asm/msr.h
> +++ b/arch/x86/include/asm/msr.h
> @@ -300,7 +300,7 @@ do {							\
>   #endif	/* !CONFIG_PARAVIRT_XXL */
>   
>   /* Instruction opcode for WRMSRNS supported in binutils >= 2.40 */
> -#define WRMSRNS _ASM_BYTES(0x0f,0x01,0xc6)
> +#define ASM_WRMSRNS _ASM_BYTES(0x0f,0x01,0xc6)
>   
>   /* Non-serializing WRMSR, when available.  Falls back to a serializing WRMSR. */
>   static __always_inline void wrmsrns(u32 msr, u64 val)
> @@ -309,7 +309,7 @@ static __always_inline void wrmsrns(u32 msr, u64 val)
>   	 * WRMSR is 2 bytes.  WRMSRNS is 3 bytes.  Pad WRMSR with a redundant
>   	 * DS prefix to avoid a trailing NOP.
>   	 */
> -	asm volatile("1: " ALTERNATIVE("ds wrmsr", WRMSRNS, X86_FEATURE_WRMSRNS)
> +	asm volatile("1: " ALTERNATIVE("ds wrmsr", ASM_WRMSRNS, X86_FEATURE_WRMSRNS)
>   		     "2: " _ASM_EXTABLE_TYPE(1b, 2b, EX_TYPE_WRMSR)
>   		     : : "c" (msr), "a" ((u32)val), "d" ((u32)(val >> 32)));
>   }

I hit the same build issue, thanks for fixing it.

Reviewed-by: Xin Li (Intel) <xin@...or.com>