linux-kernel - [PATCH 1/2] Revert "KVM: x86/mmu: Don't step down in the TDP iterator when zapping all SPTEs"

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Message-Id: <20211124214421.458549-2-mizhang@google.com>
Date:   Wed, 24 Nov 2021 21:44:20 +0000
From:   Mingwei Zhang <mizhang@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ben Gardon <bgardon@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>
Subject: [PATCH 1/2] Revert "KVM: x86/mmu: Don't step down in the TDP iterator
 when zapping all SPTEs"

Not stepping down in TDP iterator in `zap_all` case avoids re-reading the
non-leaf SPTEs, thus accelerates the zapping process . But when the number
of SPTEs is too large, we may run out of CPU time and causes a RCU stall
warnings in __handle_changed_pte() in the context of zap_gfn_range().

Revert this patch to allow eliminating RCU stall warning using a two-phase
zapping for `zap_all` case.

This reverts commit 0103098fb4f13b447b26ed514bcd3140f6791047.

Cc: Sean Christopherson <seanjc@...gle.com>
Cc: Ben Gardon <bgardon@...gle.com>
Cc: David Matlack <dmatlack@...gle.com>

Signed-off-by: Mingwei Zhang <mizhang@...gle.com>
---
 arch/x86/kvm/mmu/tdp_mmu.c | 9 +--------
 1 file changed, 1 insertion(+), 8 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 7c5dd83e52de..89d16bb104de 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -706,12 +706,6 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
 	bool zap_all = (start == 0 && end >= max_gfn_host);
 	struct tdp_iter iter;
 
-	/*
-	 * No need to try to step down in the iterator when zapping all SPTEs,
-	 * zapping the top-level non-leaf SPTEs will recurse on their children.
-	 */
-	int min_level = zap_all ? root->role.level : PG_LEVEL_4K;
-
 	/*
 	 * Bound the walk at host.MAXPHYADDR, guest accesses beyond that will
 	 * hit a #PF(RSVD) and never get to an EPT Violation/Misconfig / #NPF,
@@ -723,8 +717,7 @@ static bool zap_gfn_range(struct kvm *kvm, struct kvm_mmu_page *root,
 
 	rcu_read_lock();
 
-	for_each_tdp_pte_min_level(iter, root->spt, root->role.level,
-				   min_level, start, end) {
+	tdp_root_for_each_pte(iter, root, start, end) {
 retry:
 		if (can_yield &&
 		    tdp_mmu_iter_cond_resched(kvm, &iter, flush, shared)) {
-- 
2.34.0.rc2.393.gf8c9666880-goog