Messages in this thread Patch in this message | | | From | Sean Christopherson <> | Subject | [PATCH 1/2] KVM: x86: Add build-time assertion on usage of bit() | Date | Wed, 11 Dec 2019 09:58:21 -0800 |
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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@intel.com> ---
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); + + return 1 << (feature & 31); } static inline u8 vcpu_virt_addr_bits(struct kvm_vcpu *vcpu) -- 2.24.0
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