linux-kernel - [PATCH] KVM: VMX: Remove unnecessary INVEPT[GLOBAL] from hardware enable path

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Message-ID: <20240608001003.3296640-1-seanjc@google.com>
Date: Fri,  7 Jun 2024 17:10:03 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] KVM: VMX: Remove unnecessary INVEPT[GLOBAL] from hardware
 enable path

Remove the completely pointess global INVEPT, i.e. EPT TLB flush, from
KVM's VMX enablement path.  KVM always does a targeted TLB flush when
using a "new" EPT root, in quotes because "new" simply means a root that
isn't currently being used by the vCPU.

KVM also _deliberately_ runs with stale TLB entries for defunct roots,
i.e. doesn't do a TLB flush when vCPUs stop using roots, precisely because
KVM does the flush on first use.  As called out by the comment in
kvm_mmu_load(), the reason KVM flushes on first use is because KVM can't
guarantee the correctness of past hypervisors.

Jumping back to the global INVEPT, when the painfully terse commit
1439442c7b25 ("KVM: VMX: Enable EPT feature for KVM") was added, the
effective TLB flush being performed was:

  static void vmx_flush_tlb(struct kvm_vcpu *vcpu)
  {
          vpid_sync_vcpu_all(to_vmx(vcpu));
  }

I.e. KVM was not flushing EPT TLB entries when allocating a "new" root,
which very strongly suggests that the global INVEPT during hardware
enabling was a misguided hack that addressed the most obvious symptom,
but failed to fix the underlying bug.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/vmx/vmx.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 0e3aaf520db2..21dbe20f50ba 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -2832,9 +2832,6 @@ int vmx_hardware_enable(void)
 		return r;
 	}

-	if (enable_ept)
-		ept_sync_global();
-
 	return 0;
 }

base-commit: af0903ab52ee6d6f0f63af67fa73d5eb00f79b9a
-- 
2.45.2.505.gda0bf45e8d-goog