linux-kernel - [RFC PATCH 24/24] KVM: nSVM: Stop bombing the TLB on nested transitions

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Message-ID: <20250326194423.3717668-5-yosry.ahmed@linux.dev>
Date: Wed, 26 Mar 2025 19:44:23 +0000
From: Yosry Ahmed <yosry.ahmed@...ux.dev>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>,
	Jim Mattson <jmattson@...gle.com>,
	Maxim Levitsky <mlevitsk@...hat.com>,
	Vitaly Kuznetsov <vkuznets@...hat.com>,
	Rik van Riel <riel@...riel.com>,
	Tom Lendacky <thomas.lendacky@....com>,
	x86@...nel.org,
	kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Yosry Ahmed <yosry.ahmed@...ux.dev>
Subject: [RFC PATCH 24/24] KVM: nSVM: Stop bombing the TLB on nested transitions

Now that nested TLB flushes are properly tracked with a well-maintained
separate ASID for L2 and proper handling of L1's TLB flush requests,
drop the unconditional flushes and syncs on nested transitions.

On a Milan machine, an L1 and L2 guests were booted, both with a single
vCPU, and pinned to a single physical CPU to maximize TLB collisions. In
this setup, the cpuid_rate microbenchmark [1] showed the following
changes with this patch:

+--------+--------+-------------------+----------------------+
| L0     | L1     | cpuid_rate (base) | cpuid_rate (patched) |
+========+========+===================+======================+
| NPT    | NPT    | 256621            | 301113 (+17.3%)      |
| NPT    | Shadow | 180017            | 203347 (+12.96%)     |
| Shadow | Shadow | 177006            | 189150 (+6.86%)      |
+--------+--------+-------------------+----------------------+

[1]https://lore.kernel.org/kvm/20231109180646.2963718-1-khorenko@virtuozzo.com/

Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
Signed-off-by: Yosry Ahmed <yosry.ahmed@...ux.dev>
---
 arch/x86/kvm/svm/nested.c | 6 ------
 1 file changed, 6 deletions(-)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 0c887c91bd50d..1cc281a7b666c 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -511,9 +511,6 @@ 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 */
-	kvm_make_request(KVM_REQ_MMU_SYNC, vcpu);
-	kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
 }
 
 /* See nested_svm_entry_tlb_flush() */
@@ -525,9 +522,6 @@ static void nested_svm_exit_tlb_flush(struct kvm_vcpu *vcpu)
 
 	if (svm->nested.ctl.tlb_ctl == TLB_CONTROL_FLUSH_ALL_ASID)
 		kvm_make_request(KVM_REQ_TLB_FLUSH_GUEST, vcpu);
-
-	kvm_make_request(KVM_REQ_MMU_SYNC, vcpu);
-	kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
 }
 
 /*
-- 
2.49.0.395.g12beb8f557-goog