[1/2] KVM: x86: Add build-time assertion on usage of bit()
diff mbox series

Message ID 20191211175822.1925-2-sean.j.christopherson@intel.com
State New
Headers show
Series
  • KVM: x86: X86_FEATURE bit() cleanup
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Commit Message

Sean Christopherson Dec. 11, 2019, 5:58 p.m. UTC
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(-)

Comments

Jim Mattson Dec. 11, 2019, 6:24 p.m. UTC | #1
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
>
Sean Christopherson Dec. 11, 2019, 7:18 p.m. UTC | #2
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
> >

Patch
diff mbox series

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)