Re: [PATCH v2 09/12] x86/msr: Use the alternatives mechanism for WRMSR

[Sean Christopherson <seanjc@google.com>]

On Tue, Sep 30, 2025, Juergen Gross wrote:
> When available use one of the non-serializing WRMSR variants (WRMSRNS
> with or without an immediate operand specifying the MSR register) in
> __wrmsrq().
> 
> For the safe/unsafe variants make __wrmsrq() to be a common base
> function instead of duplicating the ALTERNATIVE*() macros. This
> requires to let native_wrmsr() use native_wrmsrq() instead of
> __wrmsrq(). While changing this, convert native_wrmsr() into an inline
> function.
> 
> Replace the only call of wsrmsrns() with the now equivalent call to
> native_wrmsrq() and remove wsrmsrns().
> 
> The paravirt case will be handled later.

...

> @@ -268,21 +357,6 @@ static inline int wrmsrq_safe(u32 msr, u64 val)
>  	return err;
>  }
>  
> -/* Instruction opcode for WRMSRNS supported in binutils >= 2.40 */
> -#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)

FYI, a use of wrmsrns() is likely coming in through the KVM (x86) tree, commit
65391feb042b ("KVM: VMX: Add host MSR read/write helpers to consolidate preemption
handling").

Probably makes sense to spin v3 after the merge window?  Or on linux-next? (I
can't tell what was used as the base, and I double-checked that the above commit
is in linux-next).

> -{
> -	/*
> -	 * 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", ASM_WRMSRNS, X86_FEATURE_WRMSRNS)
> -		     "2: " _ASM_EXTABLE_TYPE(1b, 2b, EX_TYPE_WRMSR)
> -		     : : "c" (msr), "a" ((u32)val), "d" ((u32)(val >> 32)));
> -}
> -
>  static inline void wrmsr(u32 msr, u32 low, u32 high)
>  {
>  	wrmsrq(msr, (u64)high << 32 | low);
> -- 
> 2.51.0
>