Re: [PATCH v4 4/8] KVM: VMX: Handle MMIO Stale Data in VM-Enter assembly via ALTERNATIVES_2

[Pawan Gupta]

On Thu, Oct 30, 2025 at 05:30:36PM -0700, Sean Christopherson wrote:
...
> diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
> index 1f99a98a16a2..61a809790a58 100644
> --- a/arch/x86/kvm/vmx/vmenter.S
> +++ b/arch/x86/kvm/vmx/vmenter.S
> @@ -71,6 +71,7 @@
>   * @regs:	unsigned long * (to guest registers)
>   * @flags:	VMX_RUN_VMRESUME:	use VMRESUME instead of VMLAUNCH
>   *		VMX_RUN_SAVE_SPEC_CTRL: save guest SPEC_CTRL into vmx->spec_ctrl
> + *		VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO: vCPU can access host MMIO
>   *
>   * Returns:
>   *	0 on VM-Exit, 1 on VM-Fail
> @@ -137,6 +138,12 @@ SYM_FUNC_START(__vmx_vcpu_run)
>  	/* Load @regs to RAX. */
>  	mov (%_ASM_SP), %_ASM_AX
>  
> +	/* Stash "clear for MMIO" in EFLAGS.ZF (used below). */
> +	ALTERNATIVE_2 "",								\
> +		      __stringify(test $VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO, %ebx), 	\
> +		      X86_FEATURE_CLEAR_CPU_BUF_MMIO,					\
> +		      "", X86_FEATURE_CLEAR_CPU_BUF_VM
> +
>  	/* Check if vmlaunch or vmresume is needed */
>  	bt   $VMX_RUN_VMRESUME_SHIFT, %ebx
>  
> @@ -161,7 +168,12 @@ SYM_FUNC_START(__vmx_vcpu_run)
>  	mov VCPU_RAX(%_ASM_AX), %_ASM_AX
>  
>  	/* Clobbers EFLAGS.ZF */
> -	VM_CLEAR_CPU_BUFFERS
> +	ALTERNATIVE_2 "",							\
> +		      __stringify(jz .Lskip_clear_cpu_buffers;			\
> +				  CLEAR_CPU_BUFFERS_SEQ;			\
> +				  .Lskip_clear_cpu_buffers:),			\
> +		      X86_FEATURE_CLEAR_CPU_BUF_MMIO,				\
> +		      __CLEAR_CPU_BUFFERS, X86_FEATURE_CLEAR_CPU_BUF_VM

Another way to write this could be:

	ALTERNATIVE_2 "jmp .Lskip_clear_cpu_buffers",					\
		      "jz  .Lskip_clear_cpu_buffers", X86_FEATURE_CLEAR_CPU_BUF_MMIO,	\
		      "",			      X86_FEATURE_CLEAR_CPU_BUF_VM

	CLEAR_CPU_BUFFERS_SEQ
.Lskip_clear_cpu_buffers:

With this jmp;verw; would show up in the disassembly on unaffected CPUs, I
don't know how big a problem is that. OTOH, I find this easier to understand.