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

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Message-ID: <41eb2d08-1b2d-4ca8-bcb7-f5f4611f91a9@zytor.com>
Date: Fri, 11 Apr 2025 09:18:08 -0700
From: Xin Li <xin@...or.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
        linux-hyperv@...r.kernel.org, virtualization@...ts.linux.dev,
        linux-edac@...r.kernel.org, kvm@...r.kernel.org,
        xen-devel@...ts.xenproject.org, linux-ide@...r.kernel.org,
        linux-pm@...r.kernel.org, bpf@...r.kernel.org, llvm@...ts.linux.dev,
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Subject: Re: [RFC PATCH v1 10/15] KVM: VMX: Use WRMSRNS or its immediate form
 when available

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.