Re: [PATCH] virt: IRQ bypass manager

[Alex Williamson]

On Mon, 2015-07-13 at 17:32 +0200, Eric Auger wrote:
> Hi Alex,
> On 07/10/2015 07:52 PM, Alex Williamson wrote:
> > When a physical I/O device is assigned to a virtual machine through
> > facilities like VFIO and KVM, the interrupt for the device generally
> > bounces through the host system before being injected into the VM.
> > However, hardware technologies exist that often allow the host to be
> > bypassed for some of these scenarios.  Intel Posted Interrupts allow
> > the specified physical edge interrupts to be directly injected into a
> > guest when delivered to a physical processor while the vCPU is
> > running.  ARM IRQ Forwarding allows the hypervisor to handle level
> > triggered device interrupts as edge interrupts, by giving the guest
> > control of de-asserting and unmasking the interrupt line.
> ARM IRQ Forwarding allows forwarded physical interrupts to be directly
> deactivated by the guest?

Replaced
 
> > The IRQ bypass manager here is meant to provide the shim to connect
> > interrupt producers, generally the host physical device driver, with
> > interrupt consumers, generally the hypervisor, in order to configure
> > these bypass mechanism.  To do this, we base the connection on a
> > shared, opaque token.  For KVM-VFIO this is expected to be an
> > eventfd_ctx since this is the connection we already use to connect an
> > eventfd to an irqfd on the in-kernel path.  When a producer and
> > consumer with matching tokens is found, callbacks via both registered
> > participants allow the bypass facilities to be automatically enabled.
> > 
> > Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
> > Signed-off-by: Eric Auger <eric.auger@xxxxxxxxxx>
> > ---
> > 
> > Changes:
> >  - Moved to virt/lib/
> >  - Dropped update callback
> >  - Filled in missing documentation
> >  - @resume callback renamed to @stop
> >  - Only @start/@stop are optional
> > 
> > One of the difficulties with moving this code to virt/lib is that nobody
> > builds it by default.  Thinking about this for a bit, it really needs a
> > consumer to be useful and KVM is currently the only consumer, so I tested
> > with the following:
> > 
> >  --- a/arch/x86/kvm/Kconfig
> >  +++ b/arch/x86/kvm/Kconfig
> >  @@ -100,5 +101,6 @@ config KVM_DEVICE_ASSIGNMENT
> >   # the virtualization menu.
> >   source drivers/vhost/Kconfig
> >   source drivers/lguest/Kconfig
> >  +source virt/lib/Kconfig
> >   
> >   endif # VIRTUALIZATION
> >  --- a/arch/x86/kvm/Makefile
> >  +++ b/arch/x86/kvm/Makefile
> >  @@ -20,3 +20,5 @@ kvm-amd-y		+= svm.o
> >   obj-$(CONFIG_KVM)	+= kvm.o
> >   obj-$(CONFIG_KVM_INTEL)	+= kvm-intel.o
> >   obj-$(CONFIG_KVM_AMD)	+= kvm-amd.o
> >  +
> >  +obj-y += ../../../virt/lib/
> > 
> > Perhaps if a second consumer comes along that would be justification for
> > tying it elsewhere in the build system.  ARM will obviously need to do
> > similar.  Are there better options?
> > 
> > Also, there's no maintainer for the top level virt/ directory.  Paolo,
> > would you feel comfortable taking this, maybe with some additional acks?
> > That would probably be the most convenient for merging the consumer code.
> > Thanks,
> > Alex
> > 
> >  include/linux/irqbypass.h |   90 +++++++++++++++++++
> >  virt/lib/Kconfig          |    2 
> >  virt/lib/Makefile         |    1 
> >  virt/lib/irqbypass.c      |  212 +++++++++++++++++++++++++++++++++++++++++++++
> >  4 files changed, 305 insertions(+)
> >  create mode 100644 include/linux/irqbypass.h
> >  create mode 100644 virt/lib/Kconfig
> >  create mode 100644 virt/lib/Makefile
> >  create mode 100644 virt/lib/irqbypass.c
> > 
> > diff --git a/include/linux/irqbypass.h b/include/linux/irqbypass.h
> > new file mode 100644
> > index 0000000..41df18d
> > --- /dev/null
> > +++ b/include/linux/irqbypass.h
> > @@ -0,0 +1,90 @@
> > +/*
> > + * IRQ offload/bypass manager
> > + *
> > + * Copyright (C) 2015 Red Hat, Inc.
> > + * Copyright (c) 2015 Linaro Ltd.
> > + *
> > + * This program is free software; you can redistribute it and/or modify
> > + * it under the terms of the GNU General Public License version 2 as
> > + * published by the Free Software Foundation.
> > + */
> > +#ifndef IRQBYPASS_H
> > +#define IRQBYPASS_H
> > +
> > +#include <linux/list.h>
> > +
> > +struct irq_bypass_consumer;
> > +
> > +/*
> > + * Theory of operation
> > + *
> > + * The IRQ bypass manager is a simple set of lists and callbacks that allows
> > + * IRQ producers (ex. physical interrupt sources) to be matched to IRQ
> > + * consumers (ex. virtualization hardware that allows IRQ bypass or offload)
> > + * via a shared token (ex. eventfd_ctx).  Producers and consumers register
> > + * independently.  When a token match is found, the optional @stop callback
> > + * will be called for each participant.  The pair will then be connected via
> > + * the @add_* callbacks, and finally the optional @start callback will allow
> > + * any final coordination.  When either participant is unregistered, the
> > + * process is repeated using the @del_* callbacks in place of the @add_*
> > + * callbacks.  Match tokens must be unique per producer/consumer, 1:N parings
> pairings?

Fixed

> > + * are not supported.
> > + */
> > +
> > +/**
> > + * struct irq_bypass_producer - IRQ bypass producer definition
> > + * @node: IRQ bypass manager private list management
> > + * @token: opaque token to match between producer and consumer
> > + * @irq: Linux IRQ number for the producer device
> > + * @add_consumer: Connect the IRQ producer to an IRQ consumer
> > + * @del_consumer: Disconnect the IRQ producer from an IRQ consumer
> > + * @stop: Perform any quiesce operations necessary prior to add/del (optional)
> > + * @start: Perform any startup operations necessary after add/del (optional)
> > + *
> > + * The IRQ bypass producer structure represents an interrupt source for
> > + * participation in possible host bypass, for instance an interrupt vector
> > + * for a physical device assigned to a VM.
> > + */
> > +struct irq_bypass_producer {
> > +	struct list_head node;
> > +	void *token;
> > +	int irq;
> active

See below

> > +	void (*add_consumer)(struct irq_bypass_producer *,
> > +			     struct irq_bypass_consumer *);
> > +	void (*del_consumer)(struct irq_bypass_producer *,
> > +			     struct irq_bypass_consumer *);
> > +	void (*stop)(struct irq_bypass_producer *);
> > +	void (*start)(struct irq_bypass_producer *);
> > +};
> > +
> > +/**
> > + * struct irq_bypass_consumer - IRQ bypass consumer definition
> > + * @node: IRQ bypass manager private list management
> > + * @token: opaque token to match between producer and consumer
> > + * @add_producer: Connect the IRQ consumer to an IRQ producer
> > + * @del_producer: Disconnect the IRQ consumer from an IRQ producer
> > + * @stop: Perform any quiesce operations necessary prior to add/del (optional)
> > + * @start: Perform any startup operations necessary after add/del (optional)
> > + *
> > + * The IRQ bypass consumer structure represents an interrupt sink for
> > + * participation in possible host bypass, for instance a hypervisor may
> > + * support offloads to allow bypassing the host entirely or offload
> > + * portions of the interrupt handling to the VM.
> > + */
> > +struct irq_bypass_consumer {
> > +	struct list_head node;
> > +	void *token;
> > +	void (*add_producer)(struct irq_bypass_consumer *,
> > +			     struct irq_bypass_producer *);
> > +	void (*del_producer)(struct irq_bypass_consumer *,
> > +			     struct irq_bypass_producer *);
> > +	void (*stop)(struct irq_bypass_consumer *);
> > +	void (*start)(struct irq_bypass_consumer *);
> > +};
> > +
> > +int irq_bypass_register_producer(struct irq_bypass_producer *);
> > +void irq_bypass_unregister_producer(struct irq_bypass_producer *);
> > +int irq_bypass_register_consumer(struct irq_bypass_consumer *);
> > +void irq_bypass_unregister_consumer(struct irq_bypass_consumer *);
> > +
> > +#endif /* IRQBYPASS_H */
> > diff --git a/virt/lib/Kconfig b/virt/lib/Kconfig
> > new file mode 100644
> > index 0000000..89a414f
> > --- /dev/null
> > +++ b/virt/lib/Kconfig
> > @@ -0,0 +1,2 @@
> > +config IRQ_BYPASS_MANAGER
> > +	tristate
> > diff --git a/virt/lib/Makefile b/virt/lib/Makefile
> > new file mode 100644
> > index 0000000..901228d
> > --- /dev/null
> > +++ b/virt/lib/Makefile
> > @@ -0,0 +1 @@
> > +obj-$(CONFIG_IRQ_BYPASS_MANAGER) += irqbypass.o
> > diff --git a/virt/lib/irqbypass.c b/virt/lib/irqbypass.c
> > new file mode 100644
> > index 0000000..f1091e6
> > --- /dev/null
> > +++ b/virt/lib/irqbypass.c
> > @@ -0,0 +1,212 @@
> > +/*
> > + * IRQ offload/bypass manager
> > + *
> > + * Copyright (C) 2015 Red Hat, Inc.
> > + * Copyright (c) 2015 Linaro Ltd.
> > + *
> > + * This program is free software; you can redistribute it and/or modify
> > + * it under the terms of the GNU General Public License version 2 as
> > + * published by the Free Software Foundation.
> > + *
> > + * Various virtualization hardware acceleration techniques allow bypassing
> > + * or offloading interrupts received from devices around the host kernel.
> > + * Posted Interrupts on Intel VT-d systems can allow interrupts to be
> > + * received directly by a virtual machine.  ARM IRQ Forwarding can allow
> > + * level triggered device interrupts to be de-asserted directly by the VM.
> ARM IRQ Forwarding allows forwarded physical interrupts to be directly
> deactivated by the guest

Replaced

> > + * This manager allows interrupt producers and consumers to find each other
> > + * to enable this sort of bypass.
> > + */
> > +
> > +#include <linux/irqbypass.h>
> > +#include <linux/list.h>
> > +#include <linux/module.h>
> > +#include <linux/mutex.h>
> > +
> > +MODULE_LICENSE("GPL v2");
> > +MODULE_DESCRIPTION("IRQ bypass manager utility module");
> > +
> > +static LIST_HEAD(producers);
> > +static LIST_HEAD(consumers);
> > +static DEFINE_MUTEX(lock);
> > +
> > +/* @lock must be held when calling connect */
> > +static void __connect(struct irq_bypass_producer *prod,
> > +		      struct irq_bypass_consumer *cons)
> > +{
> > +	if (prod->stop)
> > +		prod->stop(prod);
> > +	if (cons->stop)
> > +		cons->stop(cons);
> > +
> > +	prod->add_consumer(prod, cons);
> > +	cons->add_producer(cons, prod);
> 
> In case you are reluctant to add the active boolean - which looks as a
> dirty hack I acknowledge -, could we change the proto of add_* so that
> they return an error. if any of the add_* fails __connect would restore
> the initial state and return an error. list_add would not be done.
> 
> I can prototype this and add it in my forwarding series if you prefer.

The active boolean does seem like a hack, its state that either
participant should be able to generate or track themselves.  In fact, if
we add the error paths you suggest, we can determine on-demand whether a
bypass is active simply by looking for a token on both lists.  I'll post
a version with error paths.

> > +
> > +	if (cons->start)
> > +		cons->start(cons);
> > +	if (prod->start)
> > +		prod->start(prod);
> > +}
> > +
> > +/* @lock must be held when calling disconnect */
> > +static void __disconnect(struct irq_bypass_producer *prod,
> > +			 struct irq_bypass_consumer *cons)
> > +{
> > +	if (prod->stop)
> > +		prod->stop(prod);
> > +	if (cons->stop)
> > +		cons->stop(cons);
> > +
> > +	cons->del_producer(cons, prod);
> > +	prod->del_consumer(prod, cons);
> > +
> > +	if (cons->start)
> > +		cons->start(cons);
> > +	if (prod->start)
> > +		prod->start(prod);
> > +}
> > +
> > +/**
> > + * irq_bypass_register_producer - register IRQ bypass producer
> > + * @producer: pointer to producer structure
> > + *
> > + * Add the provided IRQ producer to the list of producers and connect
> > + * with any matching tokens found on the IRQ consumers list.
> token? 1-1 pairing only. not sure about the usage of plural in that case
> though. Please ignore if this is an english language mistake ;-)

Yes, singular is correct.

> > + */
> > +int irq_bypass_register_producer(struct irq_bypass_producer *producer)
> > +{
> > +	struct irq_bypass_producer *tmp;
> > +	struct irq_bypass_consumer *consumer;
> > +
> > +	if (!producer->add_consumer || !producer->del_consumer)
> > +		return -EINVAL;
> > +
> > +	might_sleep();
> > +
> > +	if (!try_module_get(THIS_MODULE))
> > +		return -ENODEV;
> > +
> > +	mutex_lock(&lock);
> > +
> > +	list_for_each_entry(tmp, &producers, node) {
> > +		if (tmp->token == producer->token) {
> > +			mutex_unlock(&lock);
> > +			module_put(THIS_MODULE);
> > +			return -EINVAL;
> > +		}
> > +	}
> > +
> > +	list_add(&producer->node, &producers);
> > +
> > +	list_for_each_entry(consumer, &consumers, node) {
> > +		if (consumer->token == producer->token) {
> > +			__connect(producer, consumer);
> > +			break;
> > +		}
> > +	}
> > +
> > +	mutex_unlock(&lock);
> > +	return 0;
> > +}
> > +EXPORT_SYMBOL_GPL(irq_bypass_register_producer);
> > +
> > +/**
> > + * irq_bypass_unregister_producer - unregister IRQ bypass producer
> > + * @producer: pointer to producer structure
> > + *
> > + * Remove a previously registered IRQ producer from the list of producers
> > + * and disconnected from any connected IRQ consumers.
> disconnect it from any connected IRQ consumer?

Yes

> > + */
> > +void irq_bypass_unregister_producer(struct irq_bypass_producer *producer)
> > +{
> > +	struct irq_bypass_consumer *consumer;
> > +
> > +	might_sleep();
> > +
> > +	mutex_lock(&lock);
> > +
> > +	list_for_each_entry(consumer, &consumers, node) {
> > +		if (consumer->token == producer->token) {
> > +			__disconnect(producer, consumer);
> > +			break;
> > +		}
> > +	}
> > +
> > +	list_del(&producer->node);
> > +
> > +	mutex_unlock(&lock);
> > +	module_put(THIS_MODULE);
> > +}
> > +EXPORT_SYMBOL_GPL(irq_bypass_unregister_producer);
> > +
> > +/**
> > + * irq_bypass_register_consumer - register IRQ bypass consumer
> > + * @consumer: pointer to consumer structure
> > + *
> > + * Add the provided IRQ consumer to the list of consumers and connect
> > + * with any matching tokens found on the IRQ producer list.
> token?

Yes

> > + */
> > +int irq_bypass_register_consumer(struct irq_bypass_consumer *consumer)
> > +{
> > +	struct irq_bypass_consumer *tmp;
> > +	struct irq_bypass_producer *producer;
> > +
> > +	if (!consumer->add_producer || !consumer->del_producer)
> > +		return -EINVAL;
> > +
> > +	might_sleep();
> > +
> > +	if (!try_module_get(THIS_MODULE))
> > +		return -ENODEV;
> > +
> > +	mutex_lock(&lock);
> > +
> > +	list_for_each_entry(tmp, &consumers, node) {
> > +		if (tmp->token == consumer->token) {
> > +			mutex_unlock(&lock);
> > +			module_put(THIS_MODULE);
> > +			return -EINVAL;
> > +		}
> > +	}
> > +
> > +	list_add(&consumer->node, &consumers);
> > +
> > +	list_for_each_entry(producer, &producers, node) {
> > +		if (producer->token == consumer->token) {
> > +			__connect(producer, consumer);
> > +			break;
> > +		}
> > +	}
> > +
> > +	mutex_unlock(&lock);
> > +	return 0;
> > +}
> > +EXPORT_SYMBOL_GPL(irq_bypass_register_consumer);
> > +
> > +/**
> > + * irq_bypass_unregister_consumer - unregister IRQ bypass consumer
> > + * @consumer: pointer to consumer structure
> > + *
> > + * Remove a previously registered IRQ consumer from the list of consumers
> > + * and disconnected from any connected IRQ producers.
> disconnect it from any connected producer.

Yep

Thanks for the review!

Alex

> > + */
> > +void irq_bypass_unregister_consumer(struct irq_bypass_consumer *consumer)
> > +{
> > +	struct irq_bypass_producer *producer;
> > +
> > +	might_sleep();
> > +
> > +	mutex_lock(&lock);
> > +
> > +	list_for_each_entry(producer, &producers, node) {
> > +		if (producer->token == consumer->token) {
> > +			__disconnect(producer, consumer);
> > +			break;
> > +		}
> > +	}
> > +
> > +	list_del(&consumer->node);
> > +
> > +	mutex_unlock(&lock);
> > +	module_put(THIS_MODULE);
> > +}
> > +EXPORT_SYMBOL_GPL(irq_bypass_unregister_consumer);
> > 
> 



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