KVM: x86: Replace guts of "governed" features with comprehensive cpu_caps

Replace the internals of the governed features framework with a more
comprehensive "guest CPU capabilities" implementation, i.e. with a guest
version of kvm_cpu_caps.  Keep the skeleton of governed features around
for now as vmx_adjust_sec_exec_control() relies on detecting governed
features to do the right thing for XSAVES, and switching all guest feature
queries to guest_cpu_cap_has() requires subtle and non-trivial changes,
i.e. is best done as a standalone change.

Tracking *all* guest capabilities that KVM cares will allow excising the
poorly named "governed features" framework, and effectively optimizes all
KVM queries of guest capabilities, i.e. doesn't require making a
subjective decision as to whether or not a feature is worth "governing",
and doesn't require adding the code to do so.

The cost of tracking all features is currently 92 bytes per vCPU on 64-bit
kernels: 100 bytes for cpu_caps versus 8 bytes for governed_features.
That cost is well worth paying even if the only benefit was eliminating
the "governed features" terminology.  And practically speaking, the real
cost is zero unless those 92 bytes pushes the size of vcpu_vmx or vcpu_svm
into a new order-N allocation, and if that happens there are better ways
to reduce the footprint of kvm_vcpu_arch, e.g. making the PMU and/or MTRR
state separate allocations.

Suggested-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Link: https://lore.kernel.org/r/20241128013424.4096668-41-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 30 insertions, 16 deletions

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index f076df9f18be..81ce8cd5814a 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -739,6 +739,23 @@ struct kvm_queued_exception {
 	bool has_payload;
 };
 
+/*
+ * Hardware-defined CPUID leafs that are either scattered by the kernel or are
+ * unknown to 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 {
+	CPUID_12_EAX	 = NCAPINTS,
+	CPUID_7_1_EDX,
+	CPUID_8000_0007_EDX,
+	CPUID_8000_0022_EAX,
+	CPUID_7_2_EDX,
+	CPUID_24_0_EBX,
+	NR_KVM_CPU_CAPS,
+
+	NKVMCAPINTS = NR_KVM_CPU_CAPS - NCAPINTS,
+};
+
 struct kvm_vcpu_arch {
 	/*
 	 * rip and regs accesses must go through
@@ -857,23 +874,20 @@ struct kvm_vcpu_arch {
 	bool is_amd_compatible;
 
 	/*
-	 * FIXME: Drop this macro and use KVM_NR_GOVERNED_FEATURES directly
-	 * when "struct kvm_vcpu_arch" is no longer defined in an
-	 * arch/x86/include/asm header.  The max is mostly arbitrary, i.e.
-	 * can be increased as necessary.
-	 */
-#define KVM_MAX_NR_GOVERNED_FEATURES BITS_PER_LONG
-
-	/*
-	 * Track whether or not the guest is allowed to use features that are
-	 * governed by KVM, where "governed" means KVM needs to manage state
-	 * and/or explicitly enable the feature in hardware.  Typically, but
-	 * not always, governed features can be used by the guest if and only
-	 * if both KVM and userspace want to expose the feature to the guest.
+	 * cpu_caps holds the effective guest capabilities, i.e. the features
+	 * the vCPU is allowed to use.  Typically, but not always, features can
+	 * be used by the guest if and only if both KVM and userspace want to
+	 * expose the feature to the guest.
+	 *
+	 * A common exception is for virtualization holes, i.e. when KVM can't
+	 * prevent the guest from using a feature, in which case the vCPU "has"
+	 * the feature regardless of what KVM or userspace desires.
+	 *
+	 * Note, features that don't require KVM involvement in any way are
+	 * NOT enforced/sanitized by KVM, i.e. are taken verbatim from the
+	 * guest CPUID provided by userspace.
 	 */
-	struct {
-		DECLARE_BITMAP(enabled, KVM_MAX_NR_GOVERNED_FEATURES);
-	} governed_features;
+	u32 cpu_caps[NR_KVM_CPU_CAPS];
 
 	u64 reserved_gpa_bits;
 	int maxphyaddr;