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

[H. Peter Anvin]

On April 11, 2025 9:18:08 AM PDT, Xin Li <x...@zytor.com> wrote:
>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.

This should be coming anyway, right?