linux-kernel - [PATCH v4 19/19] KVM: VMX: Skip VMCLEAR logic during emergency reboots if CR4.VMXE=0

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Date:   Fri, 21 Jul 2023 13:18:59 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Kai Huang <kai.huang@...el.com>, Chao Gao <chao.gao@...el.com>
Subject: [PATCH v4 19/19] KVM: VMX: Skip VMCLEAR logic during emergency
 reboots if CR4.VMXE=0

Bail from vmx_emergency_disable() without processing the list of loaded
VMCSes if CR4.VMXE=0, i.e. if the CPU can't be post-VMXON.  It should be
impossible for the list to have entries if VMX is already disabled, and
even if that invariant doesn't hold, VMCLEAR will #UD anyways, i.e.
processing the list is pointless even if it somehow isn't empty.

Assuming no existing KVM bugs, this should be a glorified nop.  The
primary motivation for the change is to avoid having code that looks like
it does VMCLEAR, but then skips VMXON, which is nonsensical.

Suggested-by: Kai Huang <kai.huang@...el.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/vmx/vmx.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 5d21931842a5..0ef5ede9cb7c 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -773,12 +773,20 @@ static void vmx_emergency_disable(void)
 
 	kvm_rebooting = true;
 
+	/*
+	 * Note, CR4.VMXE can be _cleared_ in NMI context, but it can only be
+	 * set in task context.  If this races with VMX is disabled by an NMI,
+	 * VMCLEAR and VMXOFF may #UD, but KVM will eat those faults due to
+	 * kvm_rebooting set.
+	 */
+	if (!(__read_cr4() & X86_CR4_VMXE))
+		return;
+
 	list_for_each_entry(v, &per_cpu(loaded_vmcss_on_cpu, cpu),
 			    loaded_vmcss_on_cpu_link)
 		vmcs_clear(v->vmcs);
 
-	if (__read_cr4() & X86_CR4_VMXE)
-		kvm_cpu_vmxoff();
+	kvm_cpu_vmxoff();
 }
 
 static void __loaded_vmcs_clear(void *arg)
-- 
2.41.0.487.g6d72f3e995-goog