linux-kernel - [PATCH v2 19/49] KVM: x86: Add a macro to init CPUID features that ignore host kernel support

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Date: Fri, 17 May 2024 10:38:56 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson <seanjc@...gle.com>, 
	Vitaly Kuznetsov <vkuznets@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Hou Wenlong <houwenlong.hwl@...group.com>, Kechen Lu <kechenl@...dia.com>, 
	Oliver Upton <oliver.upton@...ux.dev>, Maxim Levitsky <mlevitsk@...hat.com>, 
	Binbin Wu <binbin.wu@...ux.intel.com>, Yang Weijiang <weijiang.yang@...el.com>, 
	Robert Hoo <robert.hoo.linux@...il.com>
Subject: [PATCH v2 19/49] KVM: x86: Add a macro to init CPUID features that
 ignore host kernel support

Add a macro for use in kvm_set_cpu_caps() to automagically initialize
features that KVM wants to support based solely on the CPU's capabilities,
e.g. KVM advertises LA57 support if it's available in hardware, even if
the host kernel isn't utilizing 57-bit virtual addresses.

Take advantage of the fact that kvm_cpu_cap_mask() adjusts kvm_cpu_caps
based on raw CPUID, i.e. will clear features bits that aren't supported in
hardware, and simply force-set the capability before applying the mask.

Abusing kvm_cpu_cap_set() is a borderline evil shenanigan, but doing so
avoid extra CPUID lookups, and a future commit will harden the entire
family of *F() macros to assert (at compile time) that every feature being
allowed is part of the capability word being processed, i.e. using a macro
will bring more advantages in the future.

Avoiding CPUID also fixes a largely benign bug where KVM could incorrectly
report LA57 support on Intel CPUs whose max supported CPUID is less than 7,
i.e. if the max supported leaf (<7) happened to have bit 16 set.  In
practice, barring a funky virtual machine setup, the bug is benign as all
known CPUs that support VMX also support leaf 7.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/cpuid.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 77625a5477b1..a802c09b50ab 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -70,6 +70,18 @@ u32 xstate_required_size(u64 xstate_bv, bool compacted)
 	(boot_cpu_has(X86_FEATURE_##name) ? F(name) : 0);	\
 })

+/*
+ * Raw Feature - For features that KVM supports based purely on raw host CPUID,
+ * i.e. that KVM virtualizes even if the host kernel doesn't use the feature.
+ * Simply force set the feature in KVM's capabilities, raw CPUID support will
+ * be factored in by kvm_cpu_cap_mask().
+ */
+#define RAW_F(name)						\
+({								\
+	kvm_cpu_cap_set(X86_FEATURE_##name);			\
+	F(name);						\
+})
+
 /*
  * Magic value used by KVM when querying userspace-provided CPUID entries and
  * doesn't care about the CPIUD index because the index of the function in
@@ -682,15 +694,12 @@ void kvm_set_cpu_caps(void)
 		F(AVX512VL));

 	kvm_cpu_cap_mask(CPUID_7_ECX,
-		F(AVX512VBMI) | F(LA57) | F(PKU) | 0 /*OSPKE*/ | F(RDPID) |
+		F(AVX512VBMI) | RAW_F(LA57) | F(PKU) | 0 /*OSPKE*/ | F(RDPID) |
 		F(AVX512_VPOPCNTDQ) | F(UMIP) | F(AVX512_VBMI2) | F(GFNI) |
 		F(VAES) | F(VPCLMULQDQ) | F(AVX512_VNNI) | F(AVX512_BITALG) |
 		F(CLDEMOTE) | F(MOVDIRI) | F(MOVDIR64B) | 0 /*WAITPKG*/ |
 		F(SGX_LC) | F(BUS_LOCK_DETECT)
 	);
-	/* Set LA57 based on hardware capability. */
-	if (cpuid_ecx(7) & F(LA57))
-		kvm_cpu_cap_set(X86_FEATURE_LA57);

 	/*
 	 * PKU not yet implemented for shadow paging and requires OSPKE
-- 
2.45.0.215.g3402c0e53f-goog