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KVM: x86: Use a switch statement and macros in __feature_translate()

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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>
This commit is contained in:
Jim Mattson 2023-10-23 17:16:36 -07:00 committed by Sean Christopherson
parent eefe5e6682
commit 80c883db87
1 changed files with 12 additions and 13 deletions
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25
arch/x86/kvm/reverse_cpuid.h
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@ -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)