diff mbox series

[2/2] KVM: x86: Use a switch statement in __feature_translate()

Message ID 20231024001636.890236-2-jmattson@google.com
State New
Headers
Series [1/2] KVM: x86: Advertise CPUID.(EAX=7,ECX=2):EDX[5:0] to userspace |

Commit Message

Jim Mattson Oct. 24, 2023, 12:16 a.m. UTC 
  The compiler will probably do better than linear search.

No functional change intended.

Signed-off-by: Jim Mattson <jmattson@google.com>
---
 arch/x86/kvm/reverse_cpuid.h | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

Comments

Sean Christopherson Oct. 24, 2023, 12:25 a.m. UTC | #1 
On Mon, Oct 23, 2023, Jim Mattson wrote:
> The compiler will probably do better than linear search.

It shouldn't matter, KVM relies on the compiler to resolve the translation at
compile time, e.g. the result is fed into reverse_cpuid_check().

I.e. we should pick whatever is least ugly.
Sean Christopherson Nov. 30, 2023, 8:28 p.m. UTC | #2 
On Mon, Oct 23, 2023, Sean Christopherson wrote:
> On Mon, Oct 23, 2023, Jim Mattson wrote:
> > The compiler will probably do better than linear search.
> 
> It shouldn't matter, KVM relies on the compiler to resolve the translation at
> compile time, e.g. the result is fed into reverse_cpuid_check().
> 
> I.e. we should pick whatever is least ugly.

What if we add a macro to generate each case statement?  It's arguably a wee bit
more readable, and also eliminates the possibility of returning the wrong feature
due to copy+paste errors, e.g. nothing would break at compile time if we goofed
and did:

	case X86_FEATURE_SGX1:
		return KVM_X86_FEATURE_SGX1;
	case X86_FEATURE_SGX2:
		return KVM_X86_FEATURE_SGX1;

If you've no objection, I'll push this:

--
Author: Jim Mattson <jmattson@google.com>
Date:   Mon Oct 23 17:16:36 2023 -0700

    KVM: x86: Use a switch statement and macros in __feature_translate()
    
    Use a switch statement with macro-generated case statements to handle
    translating feature flags in order to reduce the probability of runtime
    errors due to copy+paste goofs, to make compile-time errors easier to
    debug, and to make the code more readable.
    
    E.g. the compiler won't directly generate an error for duplicate if
    statements
    
            if (x86_feature == X86_FEATURE_SGX1)
                    return KVM_X86_FEATURE_SGX1;
            else if (x86_feature == X86_FEATURE_SGX2)
                    return KVM_X86_FEATURE_SGX1;
    
    and so instead reverse_cpuid_check() will fail due to the untranslated
    entry pointing at a Linux-defined leaf, which provides practically no
    hint as to what is broken
    
      arch/x86/kvm/reverse_cpuid.h:108:2: error: call to __compiletime_assert_450 declared with 'error' attribute:
                                          BUILD_BUG_ON failed: x86_leaf == CPUID_LNX_4
              BUILD_BUG_ON(x86_leaf == CPUID_LNX_4);
              ^
    whereas duplicate case statements very explicitly point at the offending
    code:
    
      arch/x86/kvm/reverse_cpuid.h:125:2: error: duplicate case value '361'
              KVM_X86_TRANSLATE_FEATURE(SGX2);
              ^
      arch/x86/kvm/reverse_cpuid.h:124:2: error: duplicate case value '360'
              KVM_X86_TRANSLATE_FEATURE(SGX1);
              ^
    
    And without macros, the opposite type of copy+paste goof doesn't generate
    any error at compile-time, e.g. this yields no complaints:
    
            case X86_FEATURE_SGX1:
                    return KVM_X86_FEATURE_SGX1;
            case X86_FEATURE_SGX2:
                    return KVM_X86_FEATURE_SGX1;
    
    Note, __feature_translate() is forcibly inlined and the feature is known
    at compile-time, so the code generation between an if-elif sequence and a
    switch statement should be identical.
    
    Signed-off-by: Jim Mattson <jmattson@google.com>
    Link: https://lore.kernel.org/r/20231024001636.890236-2-jmattson@google.com
    [sean: use a macro, rewrite changelog]
    Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kvm/reverse_cpuid.h b/arch/x86/kvm/reverse_cpuid.h
index 17007016d8b5..aadefcaa9561 100644
--- a/arch/x86/kvm/reverse_cpuid.h
+++ b/arch/x86/kvm/reverse_cpuid.h
@@ -116,20 +116,19 @@ static __always_inline void reverse_cpuid_check(unsigned int x86_leaf)
  */
 static __always_inline u32 __feature_translate(int x86_feature)
 {
-       if (x86_feature == X86_FEATURE_SGX1)
-               return KVM_X86_FEATURE_SGX1;
-       else if (x86_feature == X86_FEATURE_SGX2)
-               return KVM_X86_FEATURE_SGX2;
-       else if (x86_feature == X86_FEATURE_SGX_EDECCSSA)
-               return KVM_X86_FEATURE_SGX_EDECCSSA;
-       else if (x86_feature == X86_FEATURE_CONSTANT_TSC)
-               return KVM_X86_FEATURE_CONSTANT_TSC;
-       else if (x86_feature == X86_FEATURE_PERFMON_V2)
-               return KVM_X86_FEATURE_PERFMON_V2;
-       else if (x86_feature == X86_FEATURE_RRSBA_CTRL)
-               return KVM_X86_FEATURE_RRSBA_CTRL;
+#define KVM_X86_TRANSLATE_FEATURE(f)   \
+       case X86_FEATURE_##f: return KVM_X86_FEATURE_##f
 
-       return x86_feature;
+       switch (x86_feature) {
+       KVM_X86_TRANSLATE_FEATURE(SGX1);
+       KVM_X86_TRANSLATE_FEATURE(SGX2);
+       KVM_X86_TRANSLATE_FEATURE(SGX_EDECCSSA);
+       KVM_X86_TRANSLATE_FEATURE(CONSTANT_TSC);
+       KVM_X86_TRANSLATE_FEATURE(PERFMON_V2);
+       KVM_X86_TRANSLATE_FEATURE(RRSBA_CTRL);
+       default:
+               return x86_feature;
+       }
 }
 
 static __always_inline u32 __feature_leaf(int x86_feature)
Jim Mattson Dec. 1, 2023, 1:39 a.m. UTC | #3 
On Thu, Nov 30, 2023 at 12:28 PM Sean Christopherson <seanjc@google.com> wrote:
>
> On Mon, Oct 23, 2023, Sean Christopherson wrote:
> > On Mon, Oct 23, 2023, Jim Mattson wrote:
> > > The compiler will probably do better than linear search.
> >
> > It shouldn't matter, KVM relies on the compiler to resolve the translation at
> > compile time, e.g. the result is fed into reverse_cpuid_check().
> >
> > I.e. we should pick whatever is least ugly.
>
> What if we add a macro to generate each case statement?  It's arguably a wee bit
> more readable, and also eliminates the possibility of returning the wrong feature
> due to copy+paste errors, e.g. nothing would break at compile time if we goofed
> and did:
>
>         case X86_FEATURE_SGX1:
>                 return KVM_X86_FEATURE_SGX1;
>         case X86_FEATURE_SGX2:
>                 return KVM_X86_FEATURE_SGX1;
>
> If you've no objection, I'll push this:

:barf:

Um, okay.

> --
> Author: Jim Mattson <jmattson@google.com>
> Date:   Mon Oct 23 17:16:36 2023 -0700
>
>     KVM: x86: Use a switch statement and macros in __feature_translate()
>
>     Use a switch statement with macro-generated case statements to handle
>     translating feature flags in order to reduce the probability of runtime
>     errors due to copy+paste goofs, to make compile-time errors easier to
>     debug, and to make the code more readable.
>
>     E.g. the compiler won't directly generate an error for duplicate if
>     statements
>
>             if (x86_feature == X86_FEATURE_SGX1)
>                     return KVM_X86_FEATURE_SGX1;
>             else if (x86_feature == X86_FEATURE_SGX2)
>                     return KVM_X86_FEATURE_SGX1;
>
>     and so instead reverse_cpuid_check() will fail due to the untranslated
>     entry pointing at a Linux-defined leaf, which provides practically no
>     hint as to what is broken
>
>       arch/x86/kvm/reverse_cpuid.h:108:2: error: call to __compiletime_assert_450 declared with 'error' attribute:
>                                           BUILD_BUG_ON failed: x86_leaf == CPUID_LNX_4
>               BUILD_BUG_ON(x86_leaf == CPUID_LNX_4);
>               ^
>     whereas duplicate case statements very explicitly point at the offending
>     code:
>
>       arch/x86/kvm/reverse_cpuid.h:125:2: error: duplicate case value '361'
>               KVM_X86_TRANSLATE_FEATURE(SGX2);
>               ^
>       arch/x86/kvm/reverse_cpuid.h:124:2: error: duplicate case value '360'
>               KVM_X86_TRANSLATE_FEATURE(SGX1);
>               ^
>
>     And without macros, the opposite type of copy+paste goof doesn't generate
>     any error at compile-time, e.g. this yields no complaints:
>
>             case X86_FEATURE_SGX1:
>                     return KVM_X86_FEATURE_SGX1;
>             case X86_FEATURE_SGX2:
>                     return KVM_X86_FEATURE_SGX1;
>
>     Note, __feature_translate() is forcibly inlined and the feature is known
>     at compile-time, so the code generation between an if-elif sequence and a
>     switch statement should be identical.
>
>     Signed-off-by: Jim Mattson <jmattson@google.com>
>     Link: https://lore.kernel.org/r/20231024001636.890236-2-jmattson@google.com
>     [sean: use a macro, rewrite changelog]
>     Signed-off-by: Sean Christopherson <seanjc@google.com>
>
> diff --git a/arch/x86/kvm/reverse_cpuid.h b/arch/x86/kvm/reverse_cpuid.h
> index 17007016d8b5..aadefcaa9561 100644
> --- a/arch/x86/kvm/reverse_cpuid.h
> +++ b/arch/x86/kvm/reverse_cpuid.h
> @@ -116,20 +116,19 @@ static __always_inline void reverse_cpuid_check(unsigned int x86_leaf)
>   */
>  static __always_inline u32 __feature_translate(int x86_feature)
>  {
> -       if (x86_feature == X86_FEATURE_SGX1)
> -               return KVM_X86_FEATURE_SGX1;
> -       else if (x86_feature == X86_FEATURE_SGX2)
> -               return KVM_X86_FEATURE_SGX2;
> -       else if (x86_feature == X86_FEATURE_SGX_EDECCSSA)
> -               return KVM_X86_FEATURE_SGX_EDECCSSA;
> -       else if (x86_feature == X86_FEATURE_CONSTANT_TSC)
> -               return KVM_X86_FEATURE_CONSTANT_TSC;
> -       else if (x86_feature == X86_FEATURE_PERFMON_V2)
> -               return KVM_X86_FEATURE_PERFMON_V2;
> -       else if (x86_feature == X86_FEATURE_RRSBA_CTRL)
> -               return KVM_X86_FEATURE_RRSBA_CTRL;
> +#define KVM_X86_TRANSLATE_FEATURE(f)   \
> +       case X86_FEATURE_##f: return KVM_X86_FEATURE_##f
>
> -       return x86_feature;
> +       switch (x86_feature) {
> +       KVM_X86_TRANSLATE_FEATURE(SGX1);
> +       KVM_X86_TRANSLATE_FEATURE(SGX2);
> +       KVM_X86_TRANSLATE_FEATURE(SGX_EDECCSSA);
> +       KVM_X86_TRANSLATE_FEATURE(CONSTANT_TSC);
> +       KVM_X86_TRANSLATE_FEATURE(PERFMON_V2);
> +       KVM_X86_TRANSLATE_FEATURE(RRSBA_CTRL);
> +       default:
> +               return x86_feature;
> +       }
>  }
>
>  static __always_inline u32 __feature_leaf(int x86_feature)
>
diff mbox series

Patch

diff --git a/arch/x86/kvm/reverse_cpuid.h b/arch/x86/kvm/reverse_cpuid.h
index 17007016d8b5..da52f5ea0351 100644
--- a/arch/x86/kvm/reverse_cpuid.h
+++ b/arch/x86/kvm/reverse_cpuid.h
@@ -116,20 +116,22 @@  static __always_inline void reverse_cpuid_check(unsigned int x86_leaf)
  */
 static __always_inline u32 __feature_translate(int x86_feature)
 {
-	if (x86_feature == X86_FEATURE_SGX1)
+	switch (x86_feature) {
+	case X86_FEATURE_SGX1:
 		return KVM_X86_FEATURE_SGX1;
-	else if (x86_feature == X86_FEATURE_SGX2)
+	case X86_FEATURE_SGX2:
 		return KVM_X86_FEATURE_SGX2;
-	else if (x86_feature == X86_FEATURE_SGX_EDECCSSA)
+	case X86_FEATURE_SGX_EDECCSSA:
 		return KVM_X86_FEATURE_SGX_EDECCSSA;
-	else if (x86_feature == X86_FEATURE_CONSTANT_TSC)
+	case X86_FEATURE_CONSTANT_TSC:
 		return KVM_X86_FEATURE_CONSTANT_TSC;
-	else if (x86_feature == X86_FEATURE_PERFMON_V2)
+	case X86_FEATURE_PERFMON_V2:
 		return KVM_X86_FEATURE_PERFMON_V2;
-	else if (x86_feature == X86_FEATURE_RRSBA_CTRL)
+	case X86_FEATURE_RRSBA_CTRL:
 		return KVM_X86_FEATURE_RRSBA_CTRL;
-
-	return x86_feature;
+	default:
+		return x86_feature;
+	}
 }
 
 static __always_inline u32 __feature_leaf(int x86_feature)