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@xxxxxxxxxx>
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@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20231024001636.890236-2-jmattson@xxxxxxxxxx
[sean: use a macro, rewrite changelog]
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
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)
