linux-kernel - [PATCH 13/28] KVM: x86/mmu: Drop RCU after processing each root in MMU notifier hooks

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Message-Id: <20211120045046.3940942-14-seanjc@google.com>
Date:   Sat, 20 Nov 2021 04:50:31 +0000
From:   Sean Christopherson <seanjc@...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,
        Hou Wenlong <houwenlong93@...ux.alibaba.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: [PATCH 13/28] KVM: x86/mmu: Drop RCU after processing each root in
 MMU notifier hooks

Drop RCU protection after processing each root when handling MMU notifier
hooks that aren't the "unmap" path, i.e. aren't zapping.  Temporarily
drop RCU to let RCU do its thing between roots, and to make it clear that
there's no special behavior that relies on holding RCU across all roots.

Currently, the RCU protection is completely superficial, it's necessary
only to make rcu_dereference() of SPTE pointers happy.  A future patch
will rely on holding RCU as a proxy for vCPUs in the guest, e.g. to
ensure shadow pages aren't freed before all vCPUs do a TLB flush (or
rather, acknowledge the need for a flush), but in that case RCU needs to
be held until the flush is complete if and only if the flush is needed
because a shadow page may have been removed.  And except for the "unmap"
path, MMU notifier events cannot remove SPs (don't toggle PRESENT bit,
and can't change the PFN for a SP).

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/mmu/tdp_mmu.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 06b500fab248..3ff7b4cd7d0e 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -1079,18 +1079,19 @@ static __always_inline bool kvm_tdp_mmu_handle_gfn(struct kvm *kvm,

 	lockdep_assert_held_write(&kvm->mmu_lock);

-	rcu_read_lock();
-
 	/*
 	 * Don't support rescheduling, none of the MMU notifiers that funnel
 	 * into this helper allow blocking; it'd be dead, wasteful code.
 	 */
 	for_each_tdp_mmu_root(kvm, root, range->slot->as_id) {
+		rcu_read_lock();
+
 		tdp_root_for_each_leaf_pte(iter, root, range->start, range->end)
 			ret |= handler(kvm, &iter, range);
+
+		rcu_read_unlock();
 	}

-	rcu_read_unlock();

 	return ret;
 }
-- 
2.34.0.rc2.393.gf8c9666880-goog