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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Mon, Mar 31, 2025 at 01:22:46AM -0700, Xin Li (Intel) wrote:
> Signed-off-by: Xin Li (Intel) <xin@xxxxxxxxx>
> ---
>  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
> +
>  .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

Is that the right path forward?

That is replace the MSR write to disable speculative execution with a
non-serialized WRMSR? Doesn't that mean the WRMSRNS is speculative?


> +     WRITE_EAX_TO_MSR_IA32_SPEC_CTRL
>  
>  .Lspec_ctrl_done:
>  
> -- 
> 2.49.0
> 
>