KVM: x86: Advertise support for WRMSRNS

[PATCH 1/2] x86/msr: Rename the WRMSRNS opcode macro to ASM_WRMSRNS (for KVM)

Posted by Sean Christopherson 11 months, 2 weeks ago

Rename the WRMSRNS instruction opcode macro so that it doesn't collide
with X86_FEATURE_WRMSRNS when using token pasting to generate references
to X86_FEATURE_WRMSRNS.  KVM heavily uses token pasting to generate KVM's
set of support feature bits, and adding WRMSRNS support in KVM will run
will run afoul of the opcode macro.

  arch/x86/kvm/cpuid.c:719:37: error: pasting "X86_FEATURE_" and "" "" does not
                                      give a valid preprocessing token
  719 |         u32 __leaf = __feature_leaf(X86_FEATURE_##name);                \
      |                                     ^~~~~~~~~~~~

KVM has worked around one such collision in the past by #undef'ing the
problematic macro in order to avoid blocking a KVM rework, but such games
are generally undesirable, e.g. requires bleeding macro details into KVM,
risks weird behavior if what KVM is #undef'ing changes, etc.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/include/asm/msr.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
index 001853541f1e..60b80a36d045 100644
--- a/arch/x86/include/asm/msr.h
+++ b/arch/x86/include/asm/msr.h
@@ -300,7 +300,7 @@ do {							\
 #endif	/* !CONFIG_PARAVIRT_XXL */
 
 /* Instruction opcode for WRMSRNS supported in binutils >= 2.40 */
-#define WRMSRNS _ASM_BYTES(0x0f,0x01,0xc6)
+#define ASM_WRMSRNS _ASM_BYTES(0x0f,0x01,0xc6)
 
 /* Non-serializing WRMSR, when available.  Falls back to a serializing WRMSR. */
 static __always_inline void wrmsrns(u32 msr, u64 val)
@@ -309,7 +309,7 @@ static __always_inline void wrmsrns(u32 msr, u64 val)
 	 * WRMSR is 2 bytes.  WRMSRNS is 3 bytes.  Pad WRMSR with a redundant
 	 * DS prefix to avoid a trailing NOP.
 	 */
-	asm volatile("1: " ALTERNATIVE("ds wrmsr", WRMSRNS, X86_FEATURE_WRMSRNS)
+	asm volatile("1: " ALTERNATIVE("ds wrmsr", ASM_WRMSRNS, X86_FEATURE_WRMSRNS)
 		     "2: " _ASM_EXTABLE_TYPE(1b, 2b, EX_TYPE_WRMSR)
 		     : : "c" (msr), "a" ((u32)val), "d" ((u32)(val >> 32)));
 }
-- 
2.48.1.711.g2feabab25a-goog

Re: [PATCH 1/2] x86/msr: Rename the WRMSRNS opcode macro to ASM_WRMSRNS (for KVM)

Posted by Xin Li 11 months, 2 weeks ago

On 2/26/2025 5:01 PM, Sean Christopherson wrote:
> Rename the WRMSRNS instruction opcode macro so that it doesn't collide
> with X86_FEATURE_WRMSRNS when using token pasting to generate references
> to X86_FEATURE_WRMSRNS.  KVM heavily uses token pasting to generate KVM's
> set of support feature bits, and adding WRMSRNS support in KVM will run
> will run afoul of the opcode macro.
> 
>    arch/x86/kvm/cpuid.c:719:37: error: pasting "X86_FEATURE_" and "" "" does not
>                                        give a valid preprocessing token
>    719 |         u32 __leaf = __feature_leaf(X86_FEATURE_##name);                \
>        |                                     ^~~~~~~~~~~~
> 
> KVM has worked around one such collision in the past by #undef'ing the
> problematic macro in order to avoid blocking a KVM rework, but such games
> are generally undesirable, e.g. requires bleeding macro details into KVM,
> risks weird behavior if what KVM is #undef'ing changes, etc.
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>   arch/x86/include/asm/msr.h | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
> index 001853541f1e..60b80a36d045 100644
> --- a/arch/x86/include/asm/msr.h
> +++ b/arch/x86/include/asm/msr.h
> @@ -300,7 +300,7 @@ do {							\
>   #endif	/* !CONFIG_PARAVIRT_XXL */
>   
>   /* Instruction opcode for WRMSRNS supported in binutils >= 2.40 */
> -#define WRMSRNS _ASM_BYTES(0x0f,0x01,0xc6)
> +#define ASM_WRMSRNS _ASM_BYTES(0x0f,0x01,0xc6)
>   
>   /* Non-serializing WRMSR, when available.  Falls back to a serializing WRMSR. */
>   static __always_inline void wrmsrns(u32 msr, u64 val)
> @@ -309,7 +309,7 @@ static __always_inline void wrmsrns(u32 msr, u64 val)
>   	 * WRMSR is 2 bytes.  WRMSRNS is 3 bytes.  Pad WRMSR with a redundant
>   	 * DS prefix to avoid a trailing NOP.
>   	 */
> -	asm volatile("1: " ALTERNATIVE("ds wrmsr", WRMSRNS, X86_FEATURE_WRMSRNS)
> +	asm volatile("1: " ALTERNATIVE("ds wrmsr", ASM_WRMSRNS, X86_FEATURE_WRMSRNS)
>   		     "2: " _ASM_EXTABLE_TYPE(1b, 2b, EX_TYPE_WRMSR)
>   		     : : "c" (msr), "a" ((u32)val), "d" ((u32)(val >> 32)));
>   }

I hit the same build issue, thanks for fixing it.

Reviewed-by: Xin Li (Intel) <xin@zytor.com>

Re: [PATCH 1/2] x86/msr: Rename the WRMSRNS opcode macro to ASM_WRMSRNS (for KVM)

Posted by Xin Li 11 months, 1 week ago

On 2/26/2025 5:14 PM, Xin Li wrote:
> On 2/26/2025 5:01 PM, Sean Christopherson wrote:
>> Rename the WRMSRNS instruction opcode macro so that it doesn't collide
>> with X86_FEATURE_WRMSRNS when using token pasting to generate references
>> to X86_FEATURE_WRMSRNS.  KVM heavily uses token pasting to generate KVM's
>> set of support feature bits, and adding WRMSRNS support in KVM will run
>> will run afoul of the opcode macro.
>>
>>    arch/x86/kvm/cpuid.c:719:37: error: pasting "X86_FEATURE_" and "" 
>> "" does not
>>                                        give a valid preprocessing token
>>    719 |         u32 __leaf = 
>> __feature_leaf(X86_FEATURE_##name);                \
>>        |                                     ^~~~~~~~~~~~
>>
>> KVM has worked around one such collision in the past by #undef'ing the
>> problematic macro in order to avoid blocking a KVM rework, but such games
>> are generally undesirable, e.g. requires bleeding macro details into KVM,
>> risks weird behavior if what KVM is #undef'ing changes, etc.
>>
>> Signed-off-by: Sean Christopherson <seanjc@google.com>
>> ---
>>   arch/x86/include/asm/msr.h | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
>> index 001853541f1e..60b80a36d045 100644
>> --- a/arch/x86/include/asm/msr.h
>> +++ b/arch/x86/include/asm/msr.h
>> @@ -300,7 +300,7 @@ do {                            \
>>   #endif    /* !CONFIG_PARAVIRT_XXL */
>>   /* Instruction opcode for WRMSRNS supported in binutils >= 2.40 */
>> -#define WRMSRNS _ASM_BYTES(0x0f,0x01,0xc6)
>> +#define ASM_WRMSRNS _ASM_BYTES(0x0f,0x01,0xc6)
>>   /* Non-serializing WRMSR, when available.  Falls back to a 
>> serializing WRMSR. */
>>   static __always_inline void wrmsrns(u32 msr, u64 val)
>> @@ -309,7 +309,7 @@ static __always_inline void wrmsrns(u32 msr, u64 val)
>>        * WRMSR is 2 bytes.  WRMSRNS is 3 bytes.  Pad WRMSR with a 
>> redundant
>>        * DS prefix to avoid a trailing NOP.
>>        */
>> -    asm volatile("1: " ALTERNATIVE("ds wrmsr", WRMSRNS, 
>> X86_FEATURE_WRMSRNS)
>> +    asm volatile("1: " ALTERNATIVE("ds wrmsr", ASM_WRMSRNS, 
>> X86_FEATURE_WRMSRNS)
>>                "2: " _ASM_EXTABLE_TYPE(1b, 2b, EX_TYPE_WRMSR)
>>                : : "c" (msr), "a" ((u32)val), "d" ((u32)(val >> 32)));
>>   }
> 
> I hit the same build issue, thanks for fixing it.
> 
> Reviewed-by: Xin Li (Intel) <xin@zytor.com>
> 

Do we need an ack from x86 maintainers?