Linux Virtualization - Oct 2014 - [RFC PATCH 1/2] Add a new register offset let interrupt reason available

From: Li Liu <john.liuli at huawei.com>

This set of patches try to implemet irqfd support of vhost-net 
based on virtio-mmio.

I had posted a mail to talking about the status of vhost-net 
on kvm-arm refer to http://www.spinics.net/lists/kvm-arm/msg10804.html.
Some dependent patches are listed in the mail too. Basically the 
vhost-net brings great performance improvements, almost 50%+.

It's easy to implement irqfd support with PCI MSI-X. But till 
now arm32 do not provide equivalent mechanism to let a device 
allocate multiple interrupts. And even the aarch64 provid LPI
but also not available in a short time.

As Gauguey Remy said "Vhost does not emulate a complete virtio 
adapter but only manage virtqueue operations". Vhost module
don't update the ISR register, so if with only one irq then it's 
no way to get the interrupt reason even we can inject the 
irq correctly.  

To get the interrupt reason to support such VIRTIO_NET_F_STATUS 
features I add a new register offset VIRTIO_MMIO_ISRMEM which 
will help to establish a shared memory region between qemu and 
virtio-mmio device. Then the interrupt reason can be accessed by
guest driver through this region. At the same time, the virtio-mmio 
dirver check this region to see irqfd is supported or not during 
the irq handler registration, and different handler will be assigned.

I want to know it's the right direction? Does it comply with the 
virtio-mmio spec.? Or anyone have more good ideas to emulate mis-x 
based on virtio-mmio? I hope to get feedback and guidance.
Thx for any help.

Li Liu (2):
  Add a new register offset let interrupt reason available
  Assign a new irq handler while irqfd enabled

 drivers/virtio/virtio_mmio.c |   55 +++++++++++++++++++++++++++++++++++++++---
 include/linux/virtio_mmio.h  |    3 +++
 2 files changed, 55 insertions(+), 3 deletions(-)

-- 
1.7.9.5

From: Li Liu <john.liuli at huawei.com>

Add a new register offset VIRTIO_MMIO_ISRMEM which help to
estblish a shared memory region between virtio-mmio driver
and qemu with two purposes:

1.Guest virtio-mmio driver can get the interrupt reason.
2.Check irqfd enabled or not to register different irq handler.

Signed-off-by: Li Liu <john.liuli at huawei.com>
---
 drivers/virtio/virtio_mmio.c |   21 ++++++++++++++++++++-
 include/linux/virtio_mmio.h  |    3 +++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/drivers/virtio/virtio_mmio.c b/drivers/virtio/virtio_mmio.c
index ef9a165..28ddb55 100644
--- a/drivers/virtio/virtio_mmio.c
+++ b/drivers/virtio/virtio_mmio.c
@@ -122,6 +122,8 @@ struct virtio_mmio_device {
 	/* a list of queues so we can dispatch IRQs */
 	spinlock_t lock;
 	struct list_head virtqueues;
+
+	uint8_t *isr_mem;
 };
 
 struct virtio_mmio_vq_info {
@@ -443,6 +445,7 @@ static int virtio_mmio_probe(struct platform_device *pdev)
 	struct virtio_mmio_device *vm_dev;
 	struct resource *mem;
 	unsigned long magic;
+	int err;
 
 	mem = platform_get_resource(pdev, IORESOURCE_MEM, 0);
 	if (!mem)
@@ -481,6 +484,15 @@ static int virtio_mmio_probe(struct platform_device *pdev)
 		return -ENXIO;
 	}
 
+	vm_dev->isr_mem = alloc_pages_exact(PAGE_SIZE, GFP_KERNEL|__GFP_ZERO);
+	if (vm_dev->isr_mem == NULL) {
+		dev_err(&pdev->dev, "Allocate isr memory failed!\n");
+		return -ENOMEM;
+	}
+
+	writel(virt_to_phys(vm_dev->isr_mem),
+	       vm_dev->base + VIRTIO_MMIO_ISRMEM);
+
 	vm_dev->vdev.id.device = readl(vm_dev->base + VIRTIO_MMIO_DEVICE_ID);
 	vm_dev->vdev.id.vendor = readl(vm_dev->base + VIRTIO_MMIO_VENDOR_ID);
 
@@ -488,13 +500,20 @@ static int virtio_mmio_probe(struct platform_device *pdev)
 
 	platform_set_drvdata(pdev, vm_dev);
 
-	return register_virtio_device(&vm_dev->vdev);
+	err = register_virtio_device(&vm_dev->vdev);
+	if (err) {
+		free_pages_exact(vm_dev->isr_mem, PAGE_SIZE);
+		vm_dev->isr_mem = NULL;
+	}
+
+	return err;
 }
 
 static int virtio_mmio_remove(struct platform_device *pdev)
 {
 	struct virtio_mmio_device *vm_dev = platform_get_drvdata(pdev);
 
+	free_pages_exact(vm_dev->isr_mem, PAGE_SIZE);
 	unregister_virtio_device(&vm_dev->vdev);
 
 	return 0;
diff --git a/include/linux/virtio_mmio.h b/include/linux/virtio_mmio.h
index 5c7b6f0..b1e3ec7 100644
--- a/include/linux/virtio_mmio.h
+++ b/include/linux/virtio_mmio.h
@@ -95,6 +95,9 @@
 /* Device status register - Read Write */
 #define VIRTIO_MMIO_STATUS		0x070
 
+/* Allocate ISRMEM for interrupt reason - Write Only */
+#define VIRTIO_MMIO_ISRMEM		0x080
+
 /* The config space is defined by each driver as
  * the per-driver configuration space - Read Write */
 #define VIRTIO_MMIO_CONFIG		0x100
-- 
1.7.9.5

From: Li Liu <john.liuli at huawei.com>

This irq handler will get the interrupt reason from a
shared memory. And will be assigned only while irqfd
enabled.

Signed-off-by: Li Liu <john.liuli at huawei.com>
---
 drivers/virtio/virtio_mmio.c |   34 ++++++++++++++++++++++++++++++++--
 1 file changed, 32 insertions(+), 2 deletions(-)

diff --git a/drivers/virtio/virtio_mmio.c b/drivers/virtio/virtio_mmio.c
index 28ddb55..7229605 100644
--- a/drivers/virtio/virtio_mmio.c
+++ b/drivers/virtio/virtio_mmio.c
@@ -259,7 +259,31 @@ static irqreturn_t vm_interrupt(int irq, void *opaque)
 	return ret;
 }
 
+/* Notify all virtqueues on an interrupt. */
+static irqreturn_t vm_interrupt_irqfd(int irq, void *opaque)
+{
+	struct virtio_mmio_device *vm_dev = opaque;
+	struct virtio_mmio_vq_info *info;
+	unsigned long status;
+	unsigned long flags;
+	irqreturn_t ret = IRQ_NONE;
 
+	/* Read the interrupt reason and reset it */
+	status = *vm_dev->isr_mem;
+	*vm_dev->isr_mem = 0x0;
+
+	if (unlikely(status & VIRTIO_MMIO_INT_CONFIG)) {
+		virtio_config_changed(&vm_dev->vdev);
+		ret = IRQ_HANDLED;
+	}
+
+	spin_lock_irqsave(&vm_dev->lock, flags);
+	list_for_each_entry(info, &vm_dev->virtqueues, node)
+		ret |= vring_interrupt(irq, info->vq);
+	spin_unlock_irqrestore(&vm_dev->lock, flags);
+
+	return ret;
+}
 
 static void vm_del_vq(struct virtqueue *vq)
 {
@@ -391,6 +415,7 @@ error_available:
 	return ERR_PTR(err);
 }
 
+#define VIRTIO_MMIO_F_IRQFD        (1 << 7)
 static int vm_find_vqs(struct virtio_device *vdev, unsigned nvqs,
 		       struct virtqueue *vqs[],
 		       vq_callback_t *callbacks[],
@@ -400,8 +425,13 @@ static int vm_find_vqs(struct virtio_device *vdev, unsigned
nvqs,
 	unsigned int irq = platform_get_irq(vm_dev->pdev, 0);
 	int i, err;
 
-	err = request_irq(irq, vm_interrupt, IRQF_SHARED,
-			dev_name(&vdev->dev), vm_dev);
+	if (*vm_dev->isr_mem & VIRTIO_MMIO_F_IRQFD) {
+		err = request_irq(irq, vm_interrupt_irqfd, IRQF_SHARED,
+				  dev_name(&vdev->dev), vm_dev);
+	} else {
+		err = request_irq(irq, vm_interrupt, IRQF_SHARED,
+				  dev_name(&vdev->dev), vm_dev);
+	}
 	if (err)
 		return err;
 
-- 
1.7.9.5

On Sat, Oct 25, 2014 at 04:24:52PM +0800, john.liuli
wrote:
> From: Li Liu <john.liuli at huawei.com>
> 
> This set of patches try to implemet irqfd support of vhost-net 
> based on virtio-mmio.
> 
> I had posted a mail to talking about the status of vhost-net 
> on kvm-arm refer to http://www.spinics.net/lists/kvm-arm/msg10804.html.
> Some dependent patches are listed in the mail too. Basically the 
> vhost-net brings great performance improvements, almost 50%+.
> 
> It's easy to implement irqfd support with PCI MSI-X. But till 
> now arm32 do not provide equivalent mechanism to let a device 
> allocate multiple interrupts. And even the aarch64 provid LPI
> but also not available in a short time.
> 
> As Gauguey Remy said "Vhost does not emulate a complete virtio 
> adapter but only manage virtqueue operations". Vhost module
> don't update the ISR register, so if with only one irq then it's 
> no way to get the interrupt reason even we can inject the 
> irq correctly.  
Well guests don't read ISR in MSI-X mode so why does it help
to set the ISR bit?
> To get the interrupt reason to support such VIRTIO_NET_F_STATUS 
> features I add a new register offset VIRTIO_MMIO_ISRMEM which 
> will help to establish a shared memory region between qemu and 
> virtio-mmio device. Then the interrupt reason can be accessed by
> guest driver through this region. At the same time, the virtio-mmio 
> dirver check this region to see irqfd is supported or not during 
> the irq handler registration, and different handler will be assigned.
> 
> I want to know it's the right direction? Does it comply with the 
> virtio-mmio spec.? Or anyone have more good ideas to emulate mis-x 
> based on virtio-mmio? I hope to get feedback and guidance.
> Thx for any help.
> 
> Li Liu (2):
>   Add a new register offset let interrupt reason available
>   Assign a new irq handler while irqfd enabled
> 
>  drivers/virtio/virtio_mmio.c |   55
+++++++++++++++++++++++++++++++++++++++---
>  include/linux/virtio_mmio.h  |    3 +++
>  2 files changed, 55 insertions(+), 3 deletions(-)
> 
> -- 
> 1.7.9.5
>

On Sat, Oct 25, 2014 at 04:24:54PM +0800, john.liuli
wrote:
> From: Li Liu <john.liuli at huawei.com>
> 
> This irq handler will get the interrupt reason from a
> shared memory. And will be assigned only while irqfd
> enabled.
> 
> Signed-off-by: Li Liu <john.liuli at huawei.com>
> ---
>  drivers/virtio/virtio_mmio.c |   34 ++++++++++++++++++++++++++++++++--
>  1 file changed, 32 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_mmio.c b/drivers/virtio/virtio_mmio.c
> index 28ddb55..7229605 100644
> --- a/drivers/virtio/virtio_mmio.c
> +++ b/drivers/virtio/virtio_mmio.c
> @@ -259,7 +259,31 @@ static irqreturn_t vm_interrupt(int irq, void *opaque)
>  	return ret;
>  }
>  
> +/* Notify all virtqueues on an interrupt. */
> +static irqreturn_t vm_interrupt_irqfd(int irq, void *opaque)
> +{
> +	struct virtio_mmio_device *vm_dev = opaque;
> +	struct virtio_mmio_vq_info *info;
> +	unsigned long status;
> +	unsigned long flags;
> +	irqreturn_t ret = IRQ_NONE;
>  
> +	/* Read the interrupt reason and reset it */
> +	status = *vm_dev->isr_mem;
> +	*vm_dev->isr_mem = 0x0;
you are reading and modifying shared memory
without atomics and any memory barriers.
Why is this safe?
> +
> +	if (unlikely(status & VIRTIO_MMIO_INT_CONFIG)) {
> +		virtio_config_changed(&vm_dev->vdev);
> +		ret = IRQ_HANDLED;
> +	}
> +
> +	spin_lock_irqsave(&vm_dev->lock, flags);
> +	list_for_each_entry(info, &vm_dev->virtqueues, node)
> +		ret |= vring_interrupt(irq, info->vq);
> +	spin_unlock_irqrestore(&vm_dev->lock, flags);
> +
> +	return ret;
> +}
>  
>  static void vm_del_vq(struct virtqueue *vq)
>  {
So you invoke callbacks for all VQs.
This won't scale well as the number of VQs grows, will it?
> @@ -391,6 +415,7 @@ error_available:
>  	return ERR_PTR(err);
>  }
>  
> +#define VIRTIO_MMIO_F_IRQFD        (1 << 7)
>  static int vm_find_vqs(struct virtio_device *vdev, unsigned nvqs,
>  		       struct virtqueue *vqs[],
>  		       vq_callback_t *callbacks[],
> @@ -400,8 +425,13 @@ static int vm_find_vqs(struct virtio_device *vdev,
unsigned nvqs,
>  	unsigned int irq = platform_get_irq(vm_dev->pdev, 0);
>  	int i, err;
>  
> -	err = request_irq(irq, vm_interrupt, IRQF_SHARED,
> -			dev_name(&vdev->dev), vm_dev);
> +	if (*vm_dev->isr_mem & VIRTIO_MMIO_F_IRQFD) {
> +		err = request_irq(irq, vm_interrupt_irqfd, IRQF_SHARED,
> +				  dev_name(&vdev->dev), vm_dev);
> +	} else {
> +		err = request_irq(irq, vm_interrupt, IRQF_SHARED,
> +				  dev_name(&vdev->dev), vm_dev);
> +	}
>  	if (err)
>  		return err;

So still a single interrupt for all VQs.
Again this doesn't scale: a single CPU has to handle
interrupts for all of them.
I think you need to find a way to get per-VQ interrupts.
> -- 
> 1.7.9.5
>

On Sat, Oct 25, 2014 at 04:24:53PM +0800, john.liuli
wrote:
> From: Li Liu <john.liuli at huawei.com>
> 
> Add a new register offset VIRTIO_MMIO_ISRMEM which help to
> estblish a shared memory region between virtio-mmio driver
> and qemu with two purposes:
> 
> 1.Guest virtio-mmio driver can get the interrupt reason.
> 2.Check irqfd enabled or not to register different irq handler.
> 
> Signed-off-by: Li Liu <john.liuli at huawei.com>
> ---
>  drivers/virtio/virtio_mmio.c |   21 ++++++++++++++++++++-
>  include/linux/virtio_mmio.h  |    3 +++
>  2 files changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/virtio/virtio_mmio.c b/drivers/virtio/virtio_mmio.c
> index ef9a165..28ddb55 100644
> --- a/drivers/virtio/virtio_mmio.c
> +++ b/drivers/virtio/virtio_mmio.c
> @@ -122,6 +122,8 @@ struct virtio_mmio_device {
>  	/* a list of queues so we can dispatch IRQs */
>  	spinlock_t lock;
>  	struct list_head virtqueues;
> +
> +	uint8_t *isr_mem;
>  };
>  
>  struct virtio_mmio_vq_info {
> @@ -443,6 +445,7 @@ static int virtio_mmio_probe(struct platform_device
*pdev)
>  	struct virtio_mmio_device *vm_dev;
>  	struct resource *mem;
>  	unsigned long magic;
> +	int err;
>  
>  	mem = platform_get_resource(pdev, IORESOURCE_MEM, 0);
>  	if (!mem)
> @@ -481,6 +484,15 @@ static int virtio_mmio_probe(struct platform_device
*pdev)
>  		return -ENXIO;
>  	}
>  
> +	vm_dev->isr_mem = alloc_pages_exact(PAGE_SIZE, GFP_KERNEL|__GFP_ZERO);
> +	if (vm_dev->isr_mem == NULL) {
> +		dev_err(&pdev->dev, "Allocate isr memory failed!\n");
> +		return -ENOMEM;
> +	}
> +
> +	writel(virt_to_phys(vm_dev->isr_mem),
> +	       vm_dev->base + VIRTIO_MMIO_ISRMEM);
> +
What happens to existing devices?
then might not expect writes at this address.
>  	vm_dev->vdev.id.device = readl(vm_dev->base +
VIRTIO_MMIO_DEVICE_ID);
>  	vm_dev->vdev.id.vendor = readl(vm_dev->base +
VIRTIO_MMIO_VENDOR_ID);
>  
> @@ -488,13 +500,20 @@ static int virtio_mmio_probe(struct platform_device
*pdev)
>  
>  	platform_set_drvdata(pdev, vm_dev);
>  
> -	return register_virtio_device(&vm_dev->vdev);
> +	err = register_virtio_device(&vm_dev->vdev);
> +	if (err) {
> +		free_pages_exact(vm_dev->isr_mem, PAGE_SIZE);
> +		vm_dev->isr_mem = NULL;
> +	}
> +
> +	return err;
>  }
>  
>  static int virtio_mmio_remove(struct platform_device *pdev)
>  {
>  	struct virtio_mmio_device *vm_dev = platform_get_drvdata(pdev);
>  
> +	free_pages_exact(vm_dev->isr_mem, PAGE_SIZE);
>  	unregister_virtio_device(&vm_dev->vdev);
>  
>  	return 0;
> diff --git a/include/linux/virtio_mmio.h b/include/linux/virtio_mmio.h
> index 5c7b6f0..b1e3ec7 100644
> --- a/include/linux/virtio_mmio.h
> +++ b/include/linux/virtio_mmio.h
> @@ -95,6 +95,9 @@
>  /* Device status register - Read Write */
>  #define VIRTIO_MMIO_STATUS		0x070
>  
> +/* Allocate ISRMEM for interrupt reason - Write Only */
> +#define VIRTIO_MMIO_ISRMEM		0x080
> +
>  /* The config space is defined by each driver as
>   * the per-driver configuration space - Read Write */
>  #define VIRTIO_MMIO_CONFIG		0x100
> -- 
> 1.7.9.5
>

On 2014/10/26 19:52, Michael S. Tsirkin wrote:
> On Sat, Oct 25, 2014 at 04:24:52PM +0800, john.liuli wrote:
>> From: Li Liu <john.liuli at huawei.com>
>>
>> This set of patches try to implemet irqfd support of vhost-net 
>> based on virtio-mmio.
>>
>> I had posted a mail to talking about the status of vhost-net 
>> on kvm-arm refer to http://www.spinics.net/lists/kvm-arm/msg10804.html.
>> Some dependent patches are listed in the mail too. Basically the 
>> vhost-net brings great performance improvements, almost 50%+.
>>
>> It's easy to implement irqfd support with PCI MSI-X. But till 
>> now arm32 do not provide equivalent mechanism to let a device 
>> allocate multiple interrupts. And even the aarch64 provid LPI
>> but also not available in a short time.
>>
>> As Gauguey Remy said "Vhost does not emulate a complete virtio 
>> adapter but only manage virtqueue operations". Vhost module
>> don't update the ISR register, so if with only one irq then
it's
>> no way to get the interrupt reason even we can inject the 
>> irq correctly.  
> 
> Well guests don't read ISR in MSI-X mode so why does it help
> to set the ISR bit?
Yeah, vhost don't need to set ISR under MSI-X mode. But for ARM
without MSI-X kind mechanism guest can't get the interrupt reason
through the only one irq hanlder (with one gsi resource).

So I build a shared memory region to provide the interrupt reason
instead of ISR regiser by qemu without bothering vhost. Then even
there's only one irq with only one irq hanlder, it still can
distinguish why irq occur.

Li.
> 
>> To get the interrupt reason to support such VIRTIO_NET_F_STATUS 
>> features I add a new register offset VIRTIO_MMIO_ISRMEM which 
>> will help to establish a shared memory region between qemu and 
>> virtio-mmio device. Then the interrupt reason can be accessed by
>> guest driver through this region. At the same time, the virtio-mmio 
>> dirver check this region to see irqfd is supported or not during 
>> the irq handler registration, and different handler will be assigned.
>>
>> I want to know it's the right direction? Does it comply with the 
>> virtio-mmio spec.? Or anyone have more good ideas to emulate mis-x 
>> based on virtio-mmio? I hope to get feedback and guidance.
>> Thx for any help.
>>
>> Li Liu (2):
>>   Add a new register offset let interrupt reason available
>>   Assign a new irq handler while irqfd enabled
>>
>>  drivers/virtio/virtio_mmio.c |   55
+++++++++++++++++++++++++++++++++++++++---
>>  include/linux/virtio_mmio.h  |    3 +++
>>  2 files changed, 55 insertions(+), 3 deletions(-)
>>
>> -- 
>> 1.7.9.5
>>
> 
> .
>

On 25 October 2014 09:24, john.liuli <john.liuli at huawei.com>
wrote:
> To get the interrupt reason to support such VIRTIO_NET_F_STATUS
> features I add a new register offset VIRTIO_MMIO_ISRMEM which
> will help to establish a shared memory region between qemu and
> virtio-mmio device. Then the interrupt reason can be accessed by
> guest driver through this region. At the same time, the virtio-mmio
> dirver check this region to see irqfd is supported or not during
> the irq handler registration, and different handler will be assigned.
If you want to add a new register you should probably propose
an update to the virtio spec. However, it seems to me it would
be better to get generic PCI/PCIe working on the ARM virt
board instead; then we can let virtio-mmio quietly fade away.
This has been on the todo list for ages (and there have been
RFC patches posted for plain PCI), it's just nobody's had time
to work on it.

thanks
-- PMM

On 2014/10/27 17:37, Peter Maydell wrote:
> On 25 October 2014 09:24, john.liuli <john.liuli at huawei.com>
wrote:
>> To get the interrupt reason to support such VIRTIO_NET_F_STATUS
>> features I add a new register offset VIRTIO_MMIO_ISRMEM which
>> will help to establish a shared memory region between qemu and
>> virtio-mmio device. Then the interrupt reason can be accessed by
>> guest driver through this region. At the same time, the virtio-mmio
>> dirver check this region to see irqfd is supported or not during
>> the irq handler registration, and different handler will be assigned.
> 
> If you want to add a new register you should probably propose
> an update to the virtio spec. However, it seems to me it would
> be better to get generic PCI/PCIe working on the ARM virt
> board instead; then we can let virtio-mmio quietly fade away.
> This has been on the todo list for ages (and there have been
> RFC patches posted for plain PCI), it's just nobody's had time
> to work on it.
> 
> thanks
> -- PMM
> 
So you mean virtio-mmio will be replaced by PCI/PCIe on ARM at last?
If so, let this patch go with the wind:). Thx.

Li.
> .
>