linux-kernel - Re: [PATCH 1/2] x86/entry: Avoid interrupt flag save and restore

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Message-ID: <2bce50fc-3cf0-eeec-1ea7-4d54550b774a@redhat.com>
Date:	Wed, 8 Jun 2016 14:34:28 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org.com>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	Rik van Riel <riel@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 1/2] x86/entry: Avoid interrupt flag save and restore



On 08/06/2016 14:16, Ingo Molnar wrote:
> > The guest ones are not quite as consistent.  I can fix that later,
> > there's no reason also to have guest context tracking split between
> > include/linux/context_tracking.h and include/linux/kvm_host.h.
>
> Could we please first do the cleanups before complicating the code and applying 
> more substantial changes?

The further cleanups wouldn't complicate the code.  It's just that
guest_enter/guest_exit require IRQs off but don't have __.

I'm thinking of something like this (untested):

diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
index d259274238db..c2dc581ddb0e 100644
--- a/include/linux/context_tracking.h
+++ b/include/linux/context_tracking.h
@@ -84,7 +84,7 @@ static inline void context_tracking_init(void) { }
 
 
 #ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
-static inline void guest_enter(void)
+static inline void __guest_enter(void)
 {
 	if (vtime_accounting_cpu_enabled())
 		vtime_guest_enter(current);
@@ -93,9 +93,19 @@ static inline void guest_enter(void)
 
 	if (context_tracking_is_enabled())
 		__context_tracking_enter(CONTEXT_GUEST);
+
+	/* KVM does not hold any references to rcu protected data when it
+	 * switches CPU into a guest mode. In fact switching to a guest mode
+	 * is very similar to exiting to userspace from rcu point of view. In
+	 * addition CPU may stay in a guest mode for quite a long time (up to
+	 * one time slice). Lets treat guest mode as quiescent state, just like
+	 * we do with user-mode execution.
+	 */
+	if (!context_tracking_cpu_is_enabled())
+		rcu_virt_note_context_switch(smp_processor_id());
 }
 
-static inline void guest_exit(void)
+static inline void __guest_exit(void)
 {
 	if (context_tracking_is_enabled())
 		__context_tracking_exit(CONTEXT_GUEST);
@@ -107,7 +117,7 @@ static inline void guest_exit(void)
 }
 
 #else
-static inline void guest_enter(void)
+static inline void __guest_enter(void)
 {
 	/*
 	 * This is running in ioctl context so its safe
@@ -118,7 +128,7 @@ static inline void guest_enter(void)
 	current->flags |= PF_VCPU;
 }
 
-static inline void guest_exit(void)
+static inline void __guest_exit(void)
 {
 	/* Flush the guest cputime we spent on the guest */
 	vtime_account_system(current);
@@ -126,4 +136,23 @@ static inline void guest_exit(void)
 }
 #endif /* CONFIG_VIRT_CPU_ACCOUNTING_GEN */
 
+static inline void guest_enter(void)
+{
+	unsigned long flags;
+
+	local_irq_save(flags);
+	__guest_enter();
+	local_irq_restore(flags);
+}
+
+/* must be called with irqs disabled */
+static inline void guest_exit(void)
+{
+	unsigned long flags;
+
+	local_irq_save(flags);
+	__guest_exit();
+	local_irq_restore(flags);
+}
+
 #endif
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 5276fe0916fc..d00fdaa8da15 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -861,40 +861,23 @@ static inline void kvm_iommu_unmap_pages(struct kvm *kvm,
 /* must be called with irqs disabled */
 static inline void __kvm_guest_enter(void)
 {
-	guest_enter();
-	/* KVM does not hold any references to rcu protected data when it
-	 * switches CPU into a guest mode. In fact switching to a guest mode
-	 * is very similar to exiting to userspace from rcu point of view. In
-	 * addition CPU may stay in a guest mode for quite a long time (up to
-	 * one time slice). Lets treat guest mode as quiescent state, just like
-	 * we do with user-mode execution.
-	 */
-	if (!context_tracking_cpu_is_enabled())
-		rcu_virt_note_context_switch(smp_processor_id());
+	__guest_enter();
 }
 
 /* must be called with irqs disabled */
 static inline void __kvm_guest_exit(void)
 {
-	guest_exit();
+	__guest_exit();
 }
 
 static inline void kvm_guest_enter(void)
 {
-	unsigned long flags;
-
-	local_irq_save(flags);
-	__kvm_guest_enter();
-	local_irq_restore(flags);
+	guest_enter();
 }
 
 static inline void kvm_guest_exit(void)
 {
-	unsigned long flags;
-
-	local_irq_save(flags);
-	__kvm_guest_exit();
-	local_irq_restore(flags);
+	guest_exit();
 }
 
 /*


and then removing the kvm_-prefixed functions.  It's little
more than code movement.

Paolo