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

Jim Mattson
2023-12-01T01:39:39Z

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)
>