[RFC PATCH 20/24] KVM: nSVM: Do not reset TLB_CONTROL in VMCB02 on nested entry

From: Yosry Ahmed
Date: Wed Mar 26 2025 - 15:47:15 EST


TLB_CONTROL is reset to TLB_CONTROL_DO_NOTHING on nested transitions to
L2 in nested_vmcb02_prepare_control(). This is unnecessary because:
- TLB_CONTROL is set in vcpu_enter_guest() if needed when servicing a
TLB flush request (by svm_flush_tlb_asid()).
- TLB_CONTROL is reset to TLB_CONTROL_DO_NOTHING after the guest is run
in svm_cpu_run().

Hence, at the point where nested_vmcb02_prepare_control() is called,
TLB_CONTROL should have already be set to TLB_CONTROL_DO_NOTHING by
svm_cpu_run() after L1 runs.

There is a TODO in nested_svm_transition_tlb_flush() about this reset
crushing pending TLB flushes. Remove it, as the reset is not really
crushing anything as explained above.

Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
Signed-off-by: Yosry Ahmed <yosry.ahmed@xxxxxxxxx>
---
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 ca8db246ac050..544913461693c 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -511,12 +511,7 @@ static void nested_svm_entry_tlb_flush(struct kvm_vcpu *vcpu)
svm->nested.last_asid = svm->nested.ctl.asid;
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);
}
@@ -710,9 +705,6 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm,
vmcb02->control.msrpm_base_pa = vmcb01->control.msrpm_base_pa;
vmcb02->control.asid = svm_nested_asid(vcpu->kvm);

- /* Also overwritten later if necessary. */
- vmcb_clr_flush_asid(vmcb02);
-
/* nested_cr3. */
if (nested_npt_enabled(svm))
nested_svm_init_mmu_context(vcpu);
--
2.49.0.395.g12beb8f557-goog