linux-kernel - [PATCH v2 09/21] KVM: VMX: Recompute "XSAVES enabled" only after CPUID update

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Message-ID: <20230729011608.1065019-10-seanjc@google.com>
Date:   Fri, 28 Jul 2023 18:15:56 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Maxim Levitsky <mlevitsk@...hat.com>
Subject: [PATCH v2 09/21] KVM: VMX: Recompute "XSAVES enabled" only after
 CPUID update

Recompute whether or not XSAVES is enabled for the guest only if the
guest's CPUID model changes instead of redoing the computation every time
KVM generates vmcs01's secondary execution controls.  The boot_cpu_has()
and cpu_has_vmx_xsaves() checks should never change after KVM is loaded,
and if they do the kernel/KVM is hosed.

Opportunistically add a comment explaining _why_ XSAVES is effectively
exposed to the guest if and only if XSAVE is also exposed to the guest.

Practically speaking, no functional change intended (KVM will do fewer
computations, but should still get the see the same xsaves_enabled value
whenever KVM looks at it).

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/vmx/vmx.c | 24 +++++++++++-------------
 1 file changed, 11 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index ca6194b0e35e..307d73749185 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -4587,19 +4587,10 @@ static u32 vmx_secondary_exec_control(struct vcpu_vmx *vmx)
 	if (!enable_pml || !atomic_read(&vcpu->kvm->nr_memslots_dirty_logging))
 		exec_control &= ~SECONDARY_EXEC_ENABLE_PML;
 
-	if (cpu_has_vmx_xsaves()) {
-		/* Exposing XSAVES only when XSAVE is exposed */
-		bool xsaves_enabled =
-			boot_cpu_has(X86_FEATURE_XSAVE) &&
-			guest_cpuid_has(vcpu, X86_FEATURE_XSAVE) &&
-			guest_cpuid_has(vcpu, X86_FEATURE_XSAVES);
-
-		vcpu->arch.xsaves_enabled = xsaves_enabled;
-
+	if (cpu_has_vmx_xsaves())
 		vmx_adjust_secondary_exec_control(vmx, &exec_control,
 						  SECONDARY_EXEC_XSAVES,
-						  xsaves_enabled, false);
-	}
+						  vcpu->arch.xsaves_enabled, false);
 
 	/*
 	 * RDPID is also gated by ENABLE_RDTSCP, turn on the control if either
@@ -7722,8 +7713,15 @@ static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 
-	/* xsaves_enabled is recomputed in vmx_compute_secondary_exec_control(). */
-	vcpu->arch.xsaves_enabled = false;
+	/*
+	 * XSAVES is effectively enabled if and only if XSAVE is also exposed
+	 * to the guest.  XSAVES depends on CR4.OSXSAVE, and CR4.OSXSAVE can be
+	 * set if and only if XSAVE is supported.
+	 */
+	vcpu->arch.xsaves_enabled = cpu_has_vmx_xsaves() &&
+				    boot_cpu_has(X86_FEATURE_XSAVE) &&
+				    guest_cpuid_has(vcpu, X86_FEATURE_XSAVE) &&
+				    guest_cpuid_has(vcpu, X86_FEATURE_XSAVES);
 
 	vmx_setup_uret_msrs(vmx);
 
-- 
2.41.0.487.g6d72f3e995-goog