[RFC PATCH v2 20/25] KVM: nSVM: Do not reset TLB_CONTROL in vmcb02 on nested VM-Enter

[Yosry Ahmed <yosry@kernel.org>]

Stop clearing TLB_CONTROL when preparing the control area of vmcb02, as
this potentially undos pending TLB flushes for L2 (e.g. through
KVM_REQ_TLB_FLUSH while L1 is running), and remove the associated TODO
comment.

This is currently harmless, because nested_svm_entry_tlb_flush() always
requests KVM_REQ_TLB_FLUSH_CURRENT on nested VM-Enter, which sets
TLB_CONTROL again before L2 is actually run. However, always flushing
will soon go away with proper TLB handling for L2, at which point always
clearing TLB_CONTROL would be a bug.

Clearing TLB_CONTROL on nested VM-Enter was probably done because
TLB_CONTROL is not cleared by the CPU on VM-Exit. However, KVM always
clears TLB_CONTROL in the active VMCB after VMRUN. Hence, at nested
VM-Enter, TLB_CONTROL in vmcb02 can only be non-zero if a TLB flush is
queued for L2 while L1 is running (i.e. KVM_REQ_TLB_FLUSH), and that
should never be ignored.

Signed-off-by: Yosry Ahmed <yosry@kernel.org>
---
 arch/x86/kvm/svm/nested.c | 10 +---------
 1 file changed, 1 insertion(+), 9 deletions(-)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index f91c22e72151e..a226aca8f9108 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -719,12 +719,7 @@ static void nested_svm_entry_tlb_flush(struct kvm_vcpu *vcpu)
 		kvm_make_request(KVM_REQ_TLB_FLUSH_GUEST, vcpu);
 	}
 
-	/*
-	 * TODO: optimize unconditional TLB flush/MMU sync.  A partial list of
-	 * things to fix before this can be conditional:
-	 *
-	 *  - Don't crush a pending TLB flush in vmcb02 on nested VMRUN
-	 */
+	/* TODO: optimize unconditional TLB flush/MMU sync */
 	kvm_make_request(KVM_REQ_MMU_SYNC, vcpu);
 	kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
 }
@@ -981,9 +976,6 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm)
 
 	vmcb02->control.asid = svm->nested.asid02;
 
-	/* Overwritten later if necessary.  */
-	vmcb_clr_flush_asid(vmcb02);
-
 	/* Use vmcb01 MMU and format if guest does not use nNPT */
 	if (nested_npt_enabled(svm)) {
 		vmcb02->control.misc_ctl &= ~SVM_MISC_ENABLE_GMET;
-- 
2.54.0.1136.gdb2ca164c4-goog