linux-kernel - [PATCH 05/13] KVM: x86/mmu: comment for_each_tdp_mmu

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Date:   Wed, 31 Mar 2021 14:08:33 -0700
From:   Ben Gardon <bgardon@...gle.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>, Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: [PATCH 05/13] KVM: x86/mmu: comment for_each_tdp_mmu_root requires
 MMU write lock

Currently, iterating over the list of TDP MMU roots can only be done
under the MMU write lock, but that will change in future commits. Add a
defensive comment to for_each_tdp_mmu_root noting that it must only be
used under the MMU lock in write mode. That function will not be
modified to work under the lock in read mode.

Signed-off-by: Ben Gardon <bgardon@...gle.com>
---
 arch/x86/kvm/mmu/tdp_mmu.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 368091adab09..365fa9f2f856 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -103,6 +103,7 @@ static inline struct kvm_mmu_page *tdp_mmu_next_root(struct kvm *kvm,
 	     tdp_mmu_next_root_valid(_kvm, _root);			\
 	     _root = tdp_mmu_next_root(_kvm, _root))
 
+/* Only safe under the MMU lock in write mode, without yielding. */
 #define for_each_tdp_mmu_root(_kvm, _root)				\
 	list_for_each_entry(_root, &_kvm->arch.tdp_mmu_roots, link)
 
-- 
2.31.0.291.g576ba9dcdaf-goog