Re: [RFC PATCH v1 10/15] KVM: VMX: Use WRMSRNS or its immediate form when available

[Sean Christopherson <seanjc@google.com>]

On Mon, Mar 31, 2025, Xin Li (Intel) wrote:
> Signed-off-by: Xin Li (Intel) <xin@zytor.com>
> ---
>  arch/x86/include/asm/msr-index.h |  6 ++++++
>  arch/x86/kvm/vmx/vmenter.S       | 28 ++++++++++++++++++++++++----
>  2 files changed, 30 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
> index e6134ef2263d..04244c3ba374 100644
> --- a/arch/x86/include/asm/msr-index.h
> +++ b/arch/x86/include/asm/msr-index.h
> @@ -1226,4 +1226,10 @@
>  						* a #GP
>  						*/
>  
> +/* Instruction opcode for WRMSRNS supported in binutils >= 2.40 */
> +#define ASM_WRMSRNS		_ASM_BYTES(0x0f,0x01,0xc6)
> +
> +/* Instruction opcode for the immediate form RDMSR/WRMSRNS */
> +#define ASM_WRMSRNS_RAX		_ASM_BYTES(0xc4,0xe7,0x7a,0xf6,0xc0)
> +
>  #endif /* _ASM_X86_MSR_INDEX_H */
> diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
> index f6986dee6f8c..9fae43723c44 100644
> --- a/arch/x86/kvm/vmx/vmenter.S
> +++ b/arch/x86/kvm/vmx/vmenter.S
> @@ -64,6 +64,29 @@
>  	RET
>  .endm
>  
> +/*
> + * Write EAX to MSR_IA32_SPEC_CTRL.
> + *
> + * Choose the best WRMSR instruction based on availability.
> + *
> + * Replace with 'wrmsrns' and 'wrmsrns %rax, $MSR_IA32_SPEC_CTRL' once binutils support them.
> + */
> +.macro WRITE_EAX_TO_MSR_IA32_SPEC_CTRL
> +	ALTERNATIVE_2 __stringify(mov $MSR_IA32_SPEC_CTRL, %ecx;		\
> +				  xor %edx, %edx;				\
> +				  mov %edi, %eax;				\
> +				  ds wrmsr),					\
> +		      __stringify(mov $MSR_IA32_SPEC_CTRL, %ecx;		\
> +				  xor %edx, %edx;				\
> +				  mov %edi, %eax;				\
> +				  ASM_WRMSRNS),					\
> +		      X86_FEATURE_WRMSRNS,					\
> +		      __stringify(xor %_ASM_AX, %_ASM_AX;			\
> +				  mov %edi, %eax;				\
> +				  ASM_WRMSRNS_RAX; .long MSR_IA32_SPEC_CTRL),	\
> +		      X86_FEATURE_MSR_IMM
> +.endm

This is quite hideous.  I have no objection to optimizing __vmx_vcpu_run(), but
I would much prefer that a macro like this live in generic code, and that it be
generic.  It should be easy enough to provide an assembly friendly equivalent to
__native_wrmsr_constant().


> +
>  .section .noinstr.text, "ax"
>  
>  /**
> @@ -123,10 +146,7 @@ SYM_FUNC_START(__vmx_vcpu_run)
>  	movl PER_CPU_VAR(x86_spec_ctrl_current), %esi
>  	cmp %edi, %esi
>  	je .Lspec_ctrl_done
> -	mov $MSR_IA32_SPEC_CTRL, %ecx
> -	xor %edx, %edx
> -	mov %edi, %eax
> -	wrmsr
> +	WRITE_EAX_TO_MSR_IA32_SPEC_CTRL
>  
>  .Lspec_ctrl_done:
>  
> -- 
> 2.49.0
>