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

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Message-ID: <39ECA4CA-9CBC-4F72-B52E-9BD06DBF9B6D@zytor.com>
Date: Fri, 11 Apr 2025 13:50:45 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Xin Li <xin@...or.com>, 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,
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Subject: Re: [RFC PATCH v1 10/15] KVM: VMX: Use WRMSRNS or its immediate form when available

On April 11, 2025 9:18:08 AM PDT, Xin Li <xin@...or.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?