linux-kernel - [PATCH v2] kvm/x86: Inform RCU of quiescent state when entering guest mode

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Date:   Wed, 11 Jul 2018 19:01:01 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     peterz@...radead.org, mhillenb@...zon.de,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        paulmck@...ux.vnet.ibm.com
Subject: [PATCH v2] kvm/x86: Inform RCU of quiescent state when entering guest mode

From: David Woodhouse <dwmw@...zon.co.uk>

RCU can spend long periods of time waiting for a CPU which is actually in
KVM guest mode, entirely pointlessly. Treat it like the idle and userspace
modes, and don't wait for it.

Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
---
 arch/x86/kvm/x86.c       | 2 ++
 include/linux/rcupdate.h | 7 +++++++
 kernel/rcu/tree.c        | 2 ++
 3 files changed, 11 insertions(+)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0046aa70205a..b0c82f70afa7 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7458,7 +7458,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 		vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD;
 	}
 
+	rcu_kvm_enter();
 	kvm_x86_ops->run(vcpu);
+	rcu_kvm_exit();
 
 	/*
 	 * Do this here before restoring debug registers on the host.  And
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 65163aa0bb04..1325f9d9ce00 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -212,6 +212,13 @@ do { \
 #error "Unknown RCU implementation specified to kernel configuration"
 #endif
 
+/*
+ * These are currently identical to the _idle_ versions but let's
+ * explicitly have separate copies to keep Paul honest in future.
+ */
+static inline void rcu_kvm_enter(void) { rcu_idle_enter(); }
+static inline void rcu_kvm_exit(void) { rcu_idle_exit(); }
+
 /*
  * The init_rcu_head_on_stack() and destroy_rcu_head_on_stack() calls
  * are needed for dynamic initialization and destruction of rcu_head
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index aa7cade1b9f3..d38c381bf4e3 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -760,6 +760,7 @@ void rcu_idle_enter(void)
 	lockdep_assert_irqs_disabled();
 	rcu_eqs_enter(false);
 }
+EXPORT_SYMBOL_GPL(rcu_idle_enter);
 
 #ifdef CONFIG_NO_HZ_FULL
 /**
@@ -913,6 +914,7 @@ void rcu_idle_exit(void)
 	rcu_eqs_exit(false);
 	local_irq_restore(flags);
 }
+EXPORT_SYMBOL_GPL(rcu_idle_exit);
 
 #ifdef CONFIG_NO_HZ_FULL
 /**
-- 
2.17.1