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);
+
+ return 1 << (feature & 31);
}
static inline u8 vcpu_virt_addr_bits(struct kvm_vcpu *vcpu)
--
2.24.0
