[PATCH v4 03/11] KVM: x86: Add support for reverse CPUID lookup of scattered features

Kai Huang
2021-04-07T20:50:14Z

From: Sean Christopherson <seanjc@google.com>

Introduce a scheme that allows KVM's CPUID magic to support features
that are scattered in the kernel's feature words. To advertise and/or
query guest support for CPUID-based features, KVM requires the bit
number of an X86_FEATURE_* to match the bit number in its associated
CPUID entry. For scattered features, this does not hold true.

Add a framework to allow defining KVM-only words, stored in
kvm_cpu_caps after the shared kernel caps, that can be used to gather
the scattered feature bits by translating X86_FEATURE_* flags into their
KVM-defined feature.

Note, because reverse_cpuid_check() effectively forces kvm_cpu_caps
lookups to be resolved at compile time, there is no runtime cost for
translating from kernel-defined to kvm-defined features.

More details here: https://lkml.kernel.org/r/X/jxCOLG+HUO4QlZ@google.com

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Kai Huang <kai.huang@intel.com>
---
arch/x86/kvm/cpuid.c | 32 +++++++++++++++++++++++++++-----
arch/x86/kvm/cpuid.h | 39 ++++++++++++++++++++++++++++++++++-----
2 files changed, 61 insertions(+), 10 deletions(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 6bd2f8b830e4..a0e7be9ed449 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -28,7 +28,7 @@
* Unlike "struct cpuinfo_x86.x86_capability", kvm_cpu_caps doesn't need to be
* aligned to sizeof(unsigned long) because it's not accessed via bitops.
*/
-u32 kvm_cpu_caps[NCAPINTS] __read_mostly;
+u32 kvm_cpu_caps[NR_KVM_CPU_CAPS] __read_mostly;
EXPORT_SYMBOL_GPL(kvm_cpu_caps);

static u32 xstate_required_size(u64 xstate_bv, bool compacted)
@@ -53,6 +53,7 @@ static u32 xstate_required_size(u64 xstate_bv, bool compacted)
}

#define F feature_bit
+#define SF(name) (boot_cpu_has(X86_FEATURE_##name) ? F(name) : 0)

static inline struct kvm_cpuid_entry2 *cpuid_entry2_find(
struct kvm_cpuid_entry2 *entries, int nent, u32 function, u32 index)
@@ -347,13 +348,13 @@ int kvm_vcpu_ioctl_get_cpuid2(struct kvm_vcpu *vcpu,
return r;
}

-static __always_inline void kvm_cpu_cap_mask(enum cpuid_leafs leaf, u32 mask)
+/* Mask kvm_cpu_caps for @leaf with the raw CPUID capabilities of this CPU. */
+static __always_inline void __kvm_cpu_cap_mask(enum cpuid_leafs leaf)
{
const struct cpuid_reg cpuid = x86_feature_cpuid(leaf * 32);
struct kvm_cpuid_entry2 entry;

reverse_cpuid_check(leaf);
- kvm_cpu_caps[leaf] &= mask;

cpuid_count(cpuid.function, cpuid.index,
&entry.eax, &entry.ebx, &entry.ecx, &entry.edx);
@@ -361,6 +362,26 @@ static __always_inline void kvm_cpu_cap_mask(enum cpuid_leafs leaf, u32 mask)
kvm_cpu_caps[leaf] &= *__cpuid_entry_get_reg(&entry, cpuid.reg);
}

+static __always_inline void kvm_cpu_cap_mask(enum cpuid_leafs leaf, u32 mask)
+{
+ /* Use the "init" variant for scattered leafs. */
+ BUILD_BUG_ON(leaf >= NCAPINTS);
+
+ kvm_cpu_caps[leaf] &= mask;
+
+ __kvm_cpu_cap_mask(leaf);
+}
+
+static __always_inline void kvm_cpu_cap_init(enum cpuid_leafs leaf, u32 mask)
+{
+ /* Use the "mask" variant for hardwared-defined leafs. */
+ BUILD_BUG_ON(leaf < NCAPINTS);
+
+ kvm_cpu_caps[leaf] = mask;
+
+ __kvm_cpu_cap_mask(leaf);
+}
+
void kvm_set_cpu_caps(void)
{
unsigned int f_nx = is_efer_nx() ? F(NX) : 0;
@@ -371,12 +392,13 @@ void kvm_set_cpu_caps(void)
unsigned int f_gbpages = 0;
unsigned int f_lm = 0;
#endif
+ memset(kvm_cpu_caps, 0, sizeof(kvm_cpu_caps));

- BUILD_BUG_ON(sizeof(kvm_cpu_caps) >
+ BUILD_BUG_ON(sizeof(kvm_cpu_caps) - (NKVMCAPINTS * sizeof(*kvm_cpu_caps)) >
sizeof(boot_cpu_data.x86_capability));

memcpy(&kvm_cpu_caps, &boot_cpu_data.x86_capability,
- sizeof(kvm_cpu_caps));
+ sizeof(kvm_cpu_caps) - (NKVMCAPINTS * sizeof(*kvm_cpu_caps)));

kvm_cpu_cap_mask(CPUID_1_ECX,
/*
diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
index 2a0c5064497f..8925a929186c 100644
--- a/arch/x86/kvm/cpuid.h
+++ b/arch/x86/kvm/cpuid.h
@@ -7,7 +7,20 @@
#include <asm/processor.h>
#include <uapi/asm/kvm_para.h>

-extern u32 kvm_cpu_caps[NCAPINTS] __read_mostly;
+/*
+ * Hardware-defined CPUID leafs that are scattered in the kernel, but need to
+ * be directly used by KVM. Note, these word values conflict with the kernel's
+ * "bug" caps, but KVM doesn't use those.
+ */
+enum kvm_only_cpuid_leafs {
+ NR_KVM_CPU_CAPS = NCAPINTS,
+
+ NKVMCAPINTS = NR_KVM_CPU_CAPS - NCAPINTS,
+};
+
+#define X86_KVM_FEATURE(w, f) ((w)*32 + (f))
+
+extern u32 kvm_cpu_caps[NR_KVM_CPU_CAPS] __read_mostly;
void kvm_set_cpu_caps(void);

void kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu);
@@ -100,6 +113,20 @@ static __always_inline void reverse_cpuid_check(unsigned int x86_leaf)
BUILD_BUG_ON(reverse_cpuid[x86_leaf].function == 0);
}

+/*
+ * Translate feature bits that are scattered in the kernel's cpufeatures word
+ * into KVM feature words that align with hardware's definitions.
+ */
+static __always_inline u32 __feature_translate(int x86_feature)
+{
+ return x86_feature;
+}
+
+static __always_inline u32 __feature_leaf(int x86_feature)
+{
+ return __feature_translate(x86_feature) / 32;
+}
+
/*
* 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
@@ -108,6 +135,8 @@ static __always_inline void reverse_cpuid_check(unsigned int x86_leaf)
*/
static __always_inline u32 __feature_bit(int x86_feature)
{
+ x86_feature = __feature_translate(x86_feature);
+
reverse_cpuid_check(x86_feature / 32);
return 1 << (x86_feature & 31);
}
@@ -116,7 +145,7 @@ static __always_inline u32 __feature_bit(int x86_feature)

static __always_inline struct cpuid_reg x86_feature_cpuid(unsigned int x86_feature)
{
- unsigned int x86_leaf = x86_feature / 32;
+ unsigned int x86_leaf = __feature_leaf(x86_feature);

reverse_cpuid_check(x86_leaf);
return reverse_cpuid[x86_leaf];
@@ -308,7 +337,7 @@ static inline bool cpuid_fault_enabled(struct kvm_vcpu *vcpu)

static __always_inline void kvm_cpu_cap_clear(unsigned int x86_feature)
{
- unsigned int x86_leaf = x86_feature / 32;
+ unsigned int x86_leaf = __feature_leaf(x86_feature);

reverse_cpuid_check(x86_leaf);
kvm_cpu_caps[x86_leaf] &= ~__feature_bit(x86_feature);
@@ -316,7 +345,7 @@ static __always_inline void kvm_cpu_cap_clear(unsigned int x86_feature)

static __always_inline void kvm_cpu_cap_set(unsigned int x86_feature)
{
- unsigned int x86_leaf = x86_feature / 32;
+ unsigned int x86_leaf = __feature_leaf(x86_feature);

reverse_cpuid_check(x86_leaf);
kvm_cpu_caps[x86_leaf] |= __feature_bit(x86_feature);
@@ -324,7 +353,7 @@ static __always_inline void kvm_cpu_cap_set(unsigned int x86_feature)

static __always_inline u32 kvm_cpu_cap_get(unsigned int x86_feature)
{
- unsigned int x86_leaf = x86_feature / 32;
+ unsigned int x86_leaf = __feature_leaf(x86_feature);

reverse_cpuid_check(x86_leaf);
return kvm_cpu_caps[x86_leaf] & __feature_bit(x86_feature);
--
2.30.2