linux-kernel - [PATCH 2/2] KVM: x86/mmu: Demote the WARN on yielded in xxx_cond_resched() to KVM_MMU_WARN

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Message-ID: <20241031170633.1502783-3-seanjc@google.com>
Date: Thu, 31 Oct 2024 10:06:33 -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, 
	David Matlack <dmatlack@...gle.com>
Subject: [PATCH 2/2] KVM: x86/mmu: Demote the WARN on yielded in
 xxx_cond_resched() to KVM_MMU_WARN_ON

Convert the WARN in tdp_mmu_iter_cond_resched() that the iterator hasn't
already yielded to a KVM_MMU_WARN_ON() so the code is compiled out for
production kernels (assuming production kernels disable KVM_PROVE_MMU).

Checking for a needed reschedule is a hot path, and KVM sanity checks
iter->yielded in several other less-hot paths, i.e. the odds of KVM not
flagging that something went sideways are quite low.  Furthermore, the
odds of KVM not noticing *and* the WARN detecting something worth
investigating are even lower.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/mmu/tdp_mmu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index a06f3d5cb651..c158ef8c1a36 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -698,7 +698,7 @@ static inline bool __must_check tdp_mmu_iter_cond_resched(struct kvm *kvm,
 							  struct tdp_iter *iter,
 							  bool flush, bool shared)
 {
-	WARN_ON_ONCE(iter->yielded);
+	KVM_MMU_WARN_ON(iter->yielded);

 	if (!need_resched() && !rwlock_needbreak(&kvm->mmu_lock))
 		return false;
-- 
2.47.0.163.g1226f6d8fa-goog