| Message ID | 20191211175822.1925-2-sean.j.christopherson@intel.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | KVM: x86: X86_FEATURE bit() cleanup | expand |
On Wed, Dec 11, 2019 at 9:58 AM Sean Christopherson <sean.j.christopherson@intel.com> 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@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); 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 >
On Wed, Dec 11, 2019 at 10:24:36AM -0800, Jim Mattson wrote: > On Wed, Dec 11, 2019 at 9:58 AM Sean Christopherson > <sean.j.christopherson@intel.com> 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@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); > > What is magical about CPUID_7_EDX? It's currently the last cpufeatures word. My thought was to force this to be updated in order to do reverse lookup on the next new word. I didn't want to use NCAPINTS because that gets updated when a new word is added to cpufeatures, i.e. wouldn't catch the case where the next new word is a Linux-defined word, which is extremely unlikely but theoretically possible. > > + > > + return 1 << (feature & 31); > > Why not BIT(feature & 31)? That's a very good question. > > } > > > > static inline u8 vcpu_virt_addr_bits(struct kvm_vcpu *vcpu) > > -- > > 2.24.0 > >
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)
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(-)