Re: [PATCH] virt: IRQ bypass manager

[Eric Auger]

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?
> 
> 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?
> + * 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
> +	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
> + * 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.

> +
> +	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 ;-)

> + */
> +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?
> + */
> +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?
> + */
> +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.

Best Regards

Eric
> + */
> +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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