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

[Xin Li]

On 4/10/2025 4:24 PM, Sean Christopherson wrote:
> > +/*
> > + * 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().

Will do.