From:   David Stevens <stevensd@chromium.org>
To:     Sean Christopherson <seanjc@google.com>,
        David Woodhouse <dwmw@amazon.co.uk>
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        David Stevens <stevensd@chromium.org>
Subject: [PATCH 4/3] KVM: x86/xen: Make runstate cache gpcs share a lock
Date:   Mon, 30 Jan 2023 14:25:19 +0900
Message-Id: <20230130052519.416881-1-stevensd@google.com>
In-Reply-To: <2d421cb18dfa1b88e5025f2f9b94e146c0858960.camel@infradead.org>
References: <2d421cb18dfa1b88e5025f2f9b94e146c0858960.camel@infradead.org>
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From: David Stevens <stevensd@chromium.org>

Simplify locking in the case where the guest's runstate_info is split
across two pages by sharing a single lock for the two gpcs.

Signed-off-by: David Stevens <stevensd@chromium.org>
---
I tested this patch with xen_shinfo_test as suggested, and it still
passes. I agree that it makes sense to do this as a seperate patch. For
the bot reported issue, looks like I forgot to build with lockdep
enabled. I'll fix the issue with that patch in the next revision of the
series, so that there aren't any commits which don't build.

 arch/x86/kvm/xen.c | 25 ++++---------------------
 1 file changed, 4 insertions(+), 21 deletions(-)

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index fa8ab23271d3..9251f88a4e0d 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -310,24 +310,10 @@ static void kvm_xen_update_runstate_guest(struct kvm_vcpu *v, bool atomic)
 			update_bit = ((void *)(&rs_times[1])) - 1;
 	} else {
 		/*
-		 * The guest's runstate_info is split across two pages and we
-		 * need to hold and validate both GPCs simultaneously. We can
-		 * declare a lock ordering GPC1 > GPC2 because nothing else
-		 * takes them more than one at a time. Set a subclass on the
-		 * gpc1 lock to make lockdep shut up about it.
+		 * The GPCs for both pages which comprise the guest's
+		 * runstate_info share a lock, and it's already locked.
 		 */
-		lock_set_subclass(gpc1->lock.dep_map, 1, _THIS_IP_);
-		if (atomic) {
-			if (!read_trylock(gpc2->lock)) {
-				read_unlock_irqrestore(gpc1->lock, flags);
-				return;
-			}
-		} else {
-			read_lock(gpc2->lock);
-		}
-
 		if (!kvm_gpc_check(gpc2, user_len2)) {
-			read_unlock(gpc2->lock);
 			read_unlock_irqrestore(gpc1->lock, flags);
 
 			/* When invoked from kvm_sched_out() we cannot sleep */
@@ -427,9 +413,6 @@ static void kvm_xen_update_runstate_guest(struct kvm_vcpu *v, bool atomic)
 		smp_wmb();
 	}
 
-	if (user_len2)
-		read_unlock(gpc2->lock);
-
 	read_unlock_irqrestore(gpc1->lock, flags);
 
 	mark_page_dirty_in_slot(v->kvm, gpc1->memslot, gpc1->gpa >> PAGE_SHIFT);
@@ -2056,8 +2039,8 @@ void kvm_xen_init_vcpu(struct kvm_vcpu *vcpu)
 
 	kvm_gpc_init(&vcpu->arch.xen.runstate_cache, vcpu->kvm, NULL,
 		     KVM_HOST_USES_PFN);
-	kvm_gpc_init(&vcpu->arch.xen.runstate2_cache, vcpu->kvm, NULL,
-		     KVM_HOST_USES_PFN);
+	kvm_gpc_init_with_lock(&vcpu->arch.xen.runstate2_cache, vcpu->kvm, NULL,
+			       KVM_HOST_USES_PFN, vcpu->arch.xen.runstate_cache.lock);
 	kvm_gpc_init(&vcpu->arch.xen.vcpu_info_cache, vcpu->kvm, NULL,
 		     KVM_HOST_USES_PFN);
 	kvm_gpc_init(&vcpu->arch.xen.vcpu_time_info_cache, vcpu->kvm, NULL,
-- 
2.39.1.456.gfc5497dd1b-goog