x86/msr: Inline rdmsr/wrmsr instructions

[PATCH v2 03/12] x86/kvm: Remove the KVM private read_msr() function

Posted by Juergen Gross 2 months, 2 weeks ago

Instead of having a KVM private read_msr() function, just use rdmsrq().

Signed-off-by: Juergen Gross <jgross@suse.com>
---
V2:
- remove the helper and use rdmsrq() directly (Sean Christopherson)
---
 arch/x86/include/asm/kvm_host.h | 10 ----------
 arch/x86/kvm/vmx/vmx.c          |  4 ++--
 2 files changed, 2 insertions(+), 12 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index f19a76d3ca0e..aed754dda1a3 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -2296,16 +2296,6 @@ static inline void kvm_load_ldt(u16 sel)
 	asm("lldt %0" : : "rm"(sel));
 }
 
-#ifdef CONFIG_X86_64
-static inline unsigned long read_msr(unsigned long msr)
-{
-	u64 value;
-
-	rdmsrq(msr, value);
-	return value;
-}
-#endif
-
 static inline void kvm_inject_gp(struct kvm_vcpu *vcpu, u32 error_code)
 {
 	kvm_queue_exception_e(vcpu, GP_VECTOR, error_code);
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index aa157fe5b7b3..12bb1769e3ae 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1288,8 +1288,8 @@ void vmx_prepare_switch_to_guest(struct kvm_vcpu *vcpu)
 	} else {
 		savesegment(fs, fs_sel);
 		savesegment(gs, gs_sel);
-		fs_base = read_msr(MSR_FS_BASE);
-		vt->msr_host_kernel_gs_base = read_msr(MSR_KERNEL_GS_BASE);
+		rdmsrq(MSR_FS_BASE, fs_base);
+		rdmsrq(MSR_KERNEL_GS_BASE, vt->msr_host_kernel_gs_base);
 	}
 
 	wrmsrq(MSR_KERNEL_GS_BASE, vmx->msr_guest_kernel_gs_base);
-- 
2.51.0

Re: [PATCH v2 03/12] x86/kvm: Remove the KVM private read_msr() function

Posted by Sean Christopherson 2 months, 2 weeks ago

For the scope:

  KVM: x86:

because x86/kvm is specifically used for guest-side code.

On Tue, Sep 30, 2025, Juergen Gross wrote:
> Instead of having a KVM private read_msr() function, just use rdmsrq().
> 
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
> V2:
> - remove the helper and use rdmsrq() directly (Sean Christopherson)
> ---
>  arch/x86/include/asm/kvm_host.h | 10 ----------
>  arch/x86/kvm/vmx/vmx.c          |  4 ++--
>  2 files changed, 2 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index f19a76d3ca0e..aed754dda1a3 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -2296,16 +2296,6 @@ static inline void kvm_load_ldt(u16 sel)
>  	asm("lldt %0" : : "rm"(sel));
>  }
>  
> -#ifdef CONFIG_X86_64
> -static inline unsigned long read_msr(unsigned long msr)
> -{
> -	u64 value;
> -
> -	rdmsrq(msr, value);
> -	return value;
> -}
> -#endif

Gah, the same commit[*] that added a wrmsrns() use also added a read_msr().  Sorry :-(

[*] 65391feb042b ("KVM: VMX: Add host MSR read/write helpers to consolidate preemption handling")

Re: [PATCH v2 03/12] x86/kvm: Remove the KVM private read_msr() function

Posted by Jürgen Groß 2 months, 2 weeks ago

On 30.09.25 18:04, Sean Christopherson wrote:
> For the scope:
> 
>    KVM: x86:
> 
> because x86/kvm is specifically used for guest-side code.

Okay, will change that.

> 
> On Tue, Sep 30, 2025, Juergen Gross wrote:
>> Instead of having a KVM private read_msr() function, just use rdmsrq().
>>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> ---
>> V2:
>> - remove the helper and use rdmsrq() directly (Sean Christopherson)
>> ---
>>   arch/x86/include/asm/kvm_host.h | 10 ----------
>>   arch/x86/kvm/vmx/vmx.c          |  4 ++--
>>   2 files changed, 2 insertions(+), 12 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index f19a76d3ca0e..aed754dda1a3 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -2296,16 +2296,6 @@ static inline void kvm_load_ldt(u16 sel)
>>   	asm("lldt %0" : : "rm"(sel));
>>   }
>>   
>> -#ifdef CONFIG_X86_64
>> -static inline unsigned long read_msr(unsigned long msr)
>> -{
>> -	u64 value;
>> -
>> -	rdmsrq(msr, value);
>> -	return value;
>> -}
>> -#endif
> 
> Gah, the same commit[*] that added a wrmsrns() use also added a read_msr().  Sorry :-(
> 
> [*] 65391feb042b ("KVM: VMX: Add host MSR read/write helpers to consolidate preemption handling")

Again, thanks for the heads up.


Juergen