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KVM: x86: Expand build-time assertion on reverse CPUID usage

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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().

No functional change intended.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Sean Christopherson 2019-12-17 13:32:41 -08:00 committed by Paolo Bonzini
parent daa0d8c3a4
commit a7c48c3f56
2 changed files with 30 additions and 6 deletions
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3
arch/x86/kvm/cpuid.c
View File

@ -281,8 +281,9 @@ int kvm_vcpu_ioctl_get_cpuid2(struct kvm_vcpu *vcpu,
return r;
}
static void cpuid_mask(u32 *word, int wordnum)
static __always_inline void cpuid_mask(u32 *word, int wordnum)
{
reverse_cpuid_check(wordnum);
*word &= boot_cpu_data.x86_capability[wordnum];
}

33
arch/x86/kvm/cpuid.h
View File

@ -56,18 +56,41 @@ static const struct cpuid_reg reverse_cpuid[] = {
[CPUID_7_1_EAX] = { 7, 1, CPUID_EAX},
};
static inline u32 bit(int bitno)
/*
* Reverse CPUID and its derivatives can only be used for hardware-defined
* feature words, i.e. words whose bits directly correspond to a CPUID leaf.
* Retrieving a feature bit or masking guest CPUID 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. And obviously
* the leaf being queried must have an entry in the lookup table.
*/
static __always_inline void reverse_cpuid_check(unsigned x86_leaf)
{
return BIT(bitno & 31);
BUILD_BUG_ON(x86_leaf == CPUID_LNX_1);
BUILD_BUG_ON(x86_leaf == CPUID_LNX_2);
BUILD_BUG_ON(x86_leaf == CPUID_LNX_3);
BUILD_BUG_ON(x86_leaf == CPUID_LNX_4);
BUILD_BUG_ON(x86_leaf >= ARRAY_SIZE(reverse_cpuid));
BUILD_BUG_ON(reverse_cpuid[x86_leaf].function == 0);
}
/*
* 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 x86_feature)
{
reverse_cpuid_check(x86_feature / 32);
return 1 << (x86_feature & 31);
}
static __always_inline struct cpuid_reg x86_feature_cpuid(unsigned x86_feature)
{
unsigned x86_leaf = x86_feature / 32;
BUILD_BUG_ON(x86_leaf >= ARRAY_SIZE(reverse_cpuid));
BUILD_BUG_ON(reverse_cpuid[x86_leaf].function == 0);
reverse_cpuid_check(x86_leaf);
return reverse_cpuid[x86_leaf];
}