Re: [PATCH 1/2] KVM: x86: Add build-time assertion on usage of bit()

[Jim Mattson]

On Wed, Dec 11, 2019 at 9:58 AM Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
>
> Add build-time checks to ensure KVM isn't trying to do a reverse CPUID
> lookup on Linux-defined feature bits, along with comments to explain
> the gory details of X86_FEATUREs and bit().
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> ---
>
> Note, the premature newline in the first line of the second comment is
> intentional to reduce churn in the next patch.
>
>  arch/x86/kvm/x86.h | 23 +++++++++++++++++++++--
>  1 file changed, 21 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index cab5e71f0f0f..4ee4175c66a7 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -144,9 +144,28 @@ static inline bool is_pae_paging(struct kvm_vcpu *vcpu)
>         return !is_long_mode(vcpu) && is_pae(vcpu) && is_paging(vcpu);
>  }
>
> -static inline u32 bit(int bitno)
> +/*
> + * Retrieve the bit mask from an X86_FEATURE_* definition.  Features contain
> + * the hardware defined bit number (stored in bits 4:0) and a software defined
> + * "word" (stored in bits 31:5).  The word is used to index into arrays of
> + * bit masks that hold the per-cpu feature capabilities, e.g. this_cpu_has().
> + */
> +static __always_inline u32 bit(int feature)
>  {
> -       return 1 << (bitno & 31);
> +       /*
> +        * bit() is intended to be used only for hardware-defined
> +        * words, i.e. words whose bits directly correspond to a CPUID leaf.
> +        * Retrieving the bit mask from a Linux-defined word is nonsensical
> +        * as the bit number/mask is an arbitrary software-defined value and
> +        * can't be used by KVM to query/control guest capabilities.
> +        */
> +       BUILD_BUG_ON((feature >> 5) == CPUID_LNX_1);
> +       BUILD_BUG_ON((feature >> 5) == CPUID_LNX_2);
> +       BUILD_BUG_ON((feature >> 5) == CPUID_LNX_3);
> +       BUILD_BUG_ON((feature >> 5) == CPUID_LNX_4);
> +       BUILD_BUG_ON((feature >> 5) > CPUID_7_EDX);

What is magical about CPUID_7_EDX?

> +
> +       return 1 << (feature & 31);

Why not BIT(feature & 31)?

>  }
>
>  static inline u8 vcpu_virt_addr_bits(struct kvm_vcpu *vcpu)
> --
> 2.24.0
>