linux-kernel - Re: [PATCH] KVM: x86: Fix implicit enum conversion goof in scattered reverse CPUID code

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Date:   Wed, 21 Apr 2021 10:10:15 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        clang-built-linux@...glegroups.com, linux-kernel@...r.kernel.org,
        Kai Huang <kai.huang@...el.com>
Subject: Re: [PATCH] KVM: x86: Fix implicit enum conversion goof in scattered
 reverse CPUID code

On Tue, Apr 20, 2021 at 06:08:50PM -0700, Sean Christopherson wrote:
> Take "enum kvm_only_cpuid_leafs" in scattered specific CPUID helpers
> (which is obvious in hindsight), and use "unsigned int" for leafs that
> can be the kernel's standard "enum cpuid_leaf" or the aforementioned
> KVM-only variant.  Loss of the enum params is a bit disapponting, but
> gcc obviously isn't providing any extra sanity checks, and the various

Unfortunately, gcc's -Wenum-conversion is behind -Wextra rather than
-Wall like clang. If you explicitly enable it with
KCFLAGS=-Wenum-conversion to your make invocation, it will warn in the
exact same way as clang:

arch/x86/kvm/cpuid.c: In function 'kvm_set_cpu_caps':
arch/x86/kvm/cpuid.c:499:29: warning: implicit conversion from 'enum kvm_only_cpuid_leafs' to 'enum cpuid_leafs' [-Wenum-conversion]
  499 |  kvm_cpu_cap_init_scattered(CPUID_12_EAX,
      |                             ^~~~~~~~~~~~
arch/x86/kvm/cpuid.c: In function '__do_cpuid_func':
arch/x86/kvm/cpuid.c:837:31: warning: implicit conversion from 'enum kvm_only_cpuid_leafs' to 'enum cpuid_leafs' [-Wenum-conversion]
  837 |   cpuid_entry_override(entry, CPUID_12_EAX);
      |                               ^~~~~~~~~~~~

clang's warning for comparison/posterity:

arch/x86/kvm/cpuid.c:499:29: warning: implicit conversion from enumeration type 'enum kvm_only_cpuid_leafs' to different enumeration type 'enum cpuid_leafs' [-Wenum-conversion]
        kvm_cpu_cap_init_scattered(CPUID_12_EAX,
        ~~~~~~~~~~~~~~~~~~~~~~~~~~ ^~~~~~~~~~~~
arch/x86/kvm/cpuid.c:837:31: warning: implicit conversion from enumeration type 'enum kvm_only_cpuid_leafs' to different enumeration type 'enum cpuid_leafs' [-Wenum-conversion]
                cpuid_entry_override(entry, CPUID_12_EAX);
                ~~~~~~~~~~~~~~~~~~~~        ^~~~~~~~~~~~
2 warnings generated.

> BUILD_BUG_ON() assertions ensure the input is in range.
> 
> This fixes implicit enum conversions that are detected by clang-11.
> 
> Fixes: 4e66c0cb79b7 ("KVM: x86: Add support for reverse CPUID lookup of scattered features")
> Cc: Kai Huang <kai.huang@...el.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

This makes GCC and clang happy in my brief testing.

I assume this will get squashed but in case not, here are some tags:

Reviewed-by: Nathan Chancellor <nathan@...nel.org>
Tested-by: Nathan Chancellor <nathan@...nel.org>

> ---
> 
> Hopefully it's not too late to squash this...
> 
>  arch/x86/kvm/cpuid.c | 5 +++--
>  arch/x86/kvm/cpuid.h | 2 +-
>  2 files changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 96e41e1a1bde..e9d644147bf5 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -365,7 +365,7 @@ int kvm_vcpu_ioctl_get_cpuid2(struct kvm_vcpu *vcpu,
>  }
>  
>  /* Mask kvm_cpu_caps for @leaf with the raw CPUID capabilities of this CPU. */
> -static __always_inline void __kvm_cpu_cap_mask(enum cpuid_leafs leaf)
> +static __always_inline void __kvm_cpu_cap_mask(unsigned int leaf)
>  {
>  	const struct cpuid_reg cpuid = x86_feature_cpuid(leaf * 32);
>  	struct kvm_cpuid_entry2 entry;
> @@ -378,7 +378,8 @@ static __always_inline void __kvm_cpu_cap_mask(enum cpuid_leafs leaf)
>  	kvm_cpu_caps[leaf] &= *__cpuid_entry_get_reg(&entry, cpuid.reg);
>  }
>  
> -static __always_inline void kvm_cpu_cap_init_scattered(enum cpuid_leafs leaf, u32 mask)
> +static __always_inline
> +void kvm_cpu_cap_init_scattered(enum kvm_only_cpuid_leafs leaf, u32 mask)
>  {
>  	/* Use kvm_cpu_cap_mask for non-scattered leafs. */
>  	BUILD_BUG_ON(leaf < NCAPINTS);
> diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
> index eeb4a3020e1b..7bb4504a2944 100644
> --- a/arch/x86/kvm/cpuid.h
> +++ b/arch/x86/kvm/cpuid.h
> @@ -236,7 +236,7 @@ static __always_inline void cpuid_entry_change(struct kvm_cpuid_entry2 *entry,
>  }
>  
>  static __always_inline void cpuid_entry_override(struct kvm_cpuid_entry2 *entry,
> -						 enum cpuid_leafs leaf)
> +						 unsigned int leaf)
>  {
>  	u32 *reg = cpuid_entry_get_reg(entry, leaf * 32);
>  
> -- 
> 2.31.1.368.gbe11c130af-goog
>