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From: "Maciej S. Szmigiero" <mail@maciej.szmigiero.name>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	Igor Mammedov <imammedo@redhat.com>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH 2/3] KVM: Use atomic_long_cmpxchg() instead of an open-coded variant
Date: Fri, 26 Nov 2021 01:31:08 +0100	[thread overview]
Message-ID: <7bdc7ee3dcc09a109cfaf9fb8662fb49ca0bec2c.1637884349.git.maciej.szmigiero@oracle.com> (raw)
In-Reply-To: <cover.1637884349.git.maciej.szmigiero@oracle.com>

From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>

Open-coding a cmpxchg()-like operation is significantly less readable than
a direct call.

Also, the open-coded version compiles to multiple instructions with
a branch on x86, instead of just a single instruction.

Since technically the open-coded variant didn't guarantee actual atomicity
add a comment there, too, that this property isn't strictly required in
this case.

Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
---
 virt/kvm/kvm_main.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index d4399db06d49..367c1cba26d2 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1378,8 +1378,12 @@ static void kvm_replace_memslot(struct kvm *kvm,
 		hash_del(&old->id_node[idx]);
 		interval_tree_remove(&old->hva_node[idx], &slots->hva_tree);
 
-		if ((long)old == atomic_long_read(&slots->last_used_slot))
-			atomic_long_set(&slots->last_used_slot, (long)new);
+		/*
+		 * The atomicity isn't strictly required here since we are
+		 * operating on an inactive memslots set anyway.
+		 */
+		atomic_long_cmpxchg(&slots->last_used_slot,
+				    (unsigned long)old, (unsigned long)new);
 
 		if (!new) {
 			kvm_erase_gfn_node(slots, old);

  parent reply	other threads:[~2021-11-26  0:33 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-11-26  0:31 [PATCH 0/3] KVM: Scalable memslots implementation additional patches Maciej S. Szmigiero
2021-11-26  0:31 ` [PATCH 1/3] KVM: x86: Avoid overflowing nr_mmu_pages in kvm_arch_commit_memory_region() Maciej S. Szmigiero
2021-11-29 21:03   ` Sean Christopherson
2021-11-26  0:31 ` Maciej S. Szmigiero [this message]
2021-11-26 10:34   ` [PATCH 2/3] KVM: Use atomic_long_cmpxchg() instead of an open-coded variant Paolo Bonzini
2021-11-26  0:31 ` [PATCH 3/3] KVM: Make kvm_for_each_memslot_in_gfn_range() strict and use iterators Maciej S. Szmigiero
2021-11-26 10:35 ` [PATCH 0/3] KVM: Scalable memslots implementation additional patches Paolo Bonzini
2021-11-26 13:18   ` Maciej S. Szmigiero
2021-11-26 13:19     ` Paolo Bonzini

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