RFC: sharing config interrupt between virtio devices for saving MSI

RFC: sharing config interrupt between virtio devices for saving MSI

[Amos Kong]

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Hi all,

I'm working on this item in Upstream Networking todolist:

| - Sharing config interrupts
|    Support more devices by sharing a single msi vector
|    between multiple virtio devices.
|    (Applies to virtio-blk too).

I have this solution here, and only have draft patches of Solution
1 & 2, let's discuss if solution 3 is feasible.

* We should not introduce perf regression in this change
* little effect is acceptable because we are _sharing_ interrupt

Solution 1:
==========
share one LSI interrupt for configuration change of all virtio devices.
Draft patch: share_lsi_for_all_config.patch

Solution 2:
==========
share one MSI interrupt for configuration change of all virtio devices.
Draft patch: share_msix_for_all_config.patch

Solution 3:
==========
dynamic adjust interrupt policy when device is added or removed
Current:
-------
- try to allocate a MSI to device's config
- try to allocate a MSI for each vq
- share one MSI for all vqs of one device
- share one LSI for config & vqs of one device

additional dynamic policies:
---------------------------
- if there is no enough MSI, adjust interrupt allocation for returning
  some MSI
   - try to find a device that allocated one MSI for each vq, change
     it to share one MSI for saving the MSI
   - try to share one MSI for config of all virtio_pci devices
   - try to share one LSI for config of all devices

- if device is removed, try to re-allocate freed MSI to existed
  devices, if they aren't in best status (one MSI for config, one
  MSI for each vq)
   - if config of all devices is sharing one LSI, try to upgrade it to MSI
   - if config of all devices is sharing one MSI, try to allocate one
     MSI for configuration change of each device
   - try to find a device that is sharing one MSI for all vqs, try to
     allocate one MSI for each vq



BTW, I saw we still notify all vqs even VIRTIO_PCI_ISR_CONFIG bit of
isr is set, is it necessary?

diff --git a/drivers/virtio/virtio_pci.c b/drivers/virtio/virtio_pci.c
index 101db3f..176aabc 100644
--- a/drivers/virtio/virtio_pci.c
+++ b/drivers/virtio/virtio_pci.c
@@ -259,9 +259,9 @@ static irqreturn_t vp_interrupt(int irq, void
*opaque)
 
        /* Configuration change?  Tell driver if it wants to know. */
        if (isr & VIRTIO_PCI_ISR_CONFIG)
-               vp_config_changed(irq, opaque);
-
-       return vp_vring_interrupt(irq, opaque);
+               return vp_config_changed(irq, opaque);
+       else
+               return vp_vring_interrupt(irq, opaque);
 }
 
 static void vp_free_vectors(struct virtio_device *vdev)

-- 
			Amos.

[-- Attachment #2: share_lsi_for_all_config.patch --]
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diff --git a/drivers/virtio/virtio_pci.c b/drivers/virtio/virtio_pci.c
index 101db3f..5ba348d 100644
--- a/drivers/virtio/virtio_pci.c
+++ b/drivers/virtio/virtio_pci.c
@@ -302,6 +302,8 @@ static void vp_free_vectors(struct virtio_device *vdev)
 	vp_dev->msix_affinity_masks = NULL;
 }
 
+//static msix_entry *config_msix_entry;
+static char config_msix_name[100];
 static int vp_request_msix_vectors(struct virtio_device *vdev, int nvectors,
 				   bool per_vq_vectors)
 {
@@ -341,14 +343,13 @@ static int vp_request_msix_vectors(struct virtio_device *vdev, int nvectors,
 
 	/* Set the vector used for configuration */
 	v = vp_dev->msix_used_vectors;
-	snprintf(vp_dev->msix_names[v], sizeof *vp_dev->msix_names,
+	snprintf(config_msix_name, sizeof *vp_dev->msix_names,
 		 "%s-config", name);
-	err = request_irq(vp_dev->msix_entries[v].vector,
-			  vp_config_changed, 0, vp_dev->msix_names[v],
-			  vp_dev);
+	err = request_irq(vp_dev->pci_dev->irq, vp_config_changed, IRQF_SHARED, config_msix_name, vp_dev);
 	if (err)
 		goto error;
-	++vp_dev->msix_used_vectors;
+
+        //++vp_dev->msix_used_vectors;
 
 	iowrite16(v, vp_dev->ioaddr + VIRTIO_MSI_CONFIG_VECTOR);
 	/* Verify we had enough resources to assign the vector */
@@ -380,7 +381,7 @@ static int vp_request_intx(struct virtio_device *vdev)
 {
 	int err;
 	struct virtio_pci_device *vp_dev = to_vp_device(vdev);
-
+	printk(KERN_INFO "share irq: %d\n", vp_dev->pci_dev->irq);
 	err = request_irq(vp_dev->pci_dev->irq, vp_interrupt,
 			  IRQF_SHARED, dev_name(&vdev->dev), vp_dev);
 	if (!err)
@@ -536,13 +537,13 @@ static int vp_try_to_find_vqs(struct virtio_device *vdev, unsigned nvqs,
 	} else {
 		if (per_vq_vectors) {
 			/* Best option: one for change interrupt, one per vq. */
-			nvectors = 1;
+			nvectors = 0;
 			for (i = 0; i < nvqs; ++i)
 				if (callbacks[i])
 					++nvectors;
 		} else {
 			/* Second best: one for change, shared for all vqs. */
-			nvectors = 2;
+			nvectors = 1;
 		}
 
 		err = vp_request_msix_vectors(vdev, nvectors, per_vq_vectors);

[-- Attachment #3: share_msix_for_all_config.path --]
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diff --git a/drivers/virtio/virtio_pci.c b/drivers/virtio/virtio_pci.c
index 101db3f..5ae1844 100644
--- a/drivers/virtio/virtio_pci.c
+++ b/drivers/virtio/virtio_pci.c
@@ -62,6 +62,8 @@ struct virtio_pci_device
 
 	/* Whether we have vector per vq */
 	bool per_vq_vectors;
+
+	char config_msix_name[256];
 };
 
 /* Constants for MSI-X */
@@ -302,6 +304,8 @@ static void vp_free_vectors(struct virtio_device *vdev)
 	vp_dev->msix_affinity_masks = NULL;
 }
 
+static struct msix_entry *config_msix_entry;
+static char *config_msix_name;
 static int vp_request_msix_vectors(struct virtio_device *vdev, int nvectors,
 				   bool per_vq_vectors)
 {
@@ -309,9 +313,14 @@ static int vp_request_msix_vectors(struct virtio_device *vdev, int nvectors,
 	const char *name = dev_name(&vp_dev->vdev.dev);
 	unsigned i, v;
 	int err = -ENOMEM;
+        int has_config_msix = 1;
 
-	vp_dev->msix_vectors = nvectors;
+        if (!config_msix_entry) {
+		has_config_msix = 0;
+		nvectors += 1;
+        }
 
+	vp_dev->msix_vectors = nvectors;
 	vp_dev->msix_entries = kmalloc(nvectors * sizeof *vp_dev->msix_entries,
 				       GFP_KERNEL);
 	if (!vp_dev->msix_entries)
@@ -341,14 +350,24 @@ static int vp_request_msix_vectors(struct virtio_device *vdev, int nvectors,
 
 	/* Set the vector used for configuration */
 	v = vp_dev->msix_used_vectors;
-	snprintf(vp_dev->msix_names[v], sizeof *vp_dev->msix_names,
+
+        if (has_config_msix == 0) {
+		config_msix_entry = &vp_dev->msix_entries[v];
+		config_msix_name = vp_dev->msix_names[v];
+        } else {
+		config_msix_name = vp_dev->config_msix_name;
+	}
+
+	snprintf(vp_dev->config_msix_name, sizeof *vp_dev->msix_names,
 		 "%s-config", name);
-	err = request_irq(vp_dev->msix_entries[v].vector,
-			  vp_config_changed, 0, vp_dev->msix_names[v],
-			  vp_dev);
+	err = request_irq(config_msix_entry->vector,
+                          vp_config_changed, IRQF_SHARED,
+                          vp_dev->config_msix_name, vp_dev);
 	if (err)
 		goto error;
-	++vp_dev->msix_used_vectors;
+
+        if (has_config_msix == 0)
+             ++vp_dev->msix_used_vectors;
 
 	iowrite16(v, vp_dev->ioaddr + VIRTIO_MSI_CONFIG_VECTOR);
 	/* Verify we had enough resources to assign the vector */
@@ -536,13 +555,13 @@ static int vp_try_to_find_vqs(struct virtio_device *vdev, unsigned nvqs,
 	} else {
 		if (per_vq_vectors) {
 			/* Best option: one for change interrupt, one per vq. */
-			nvectors = 1;
+			nvectors = 0;
 			for (i = 0; i < nvqs; ++i)
 				if (callbacks[i])
 					++nvectors;
 		} else {
 			/* Second best: one for change, shared for all vqs. */
-			nvectors = 2;
+			nvectors = 1;
 		}
 
 		err = vp_request_msix_vectors(vdev, nvectors, per_vq_vectors);

[-- Attachment #4: Type: text/plain, Size: 183 bytes --]

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Re: RFC: sharing config interrupt between virtio devices for saving MSI

[Amos Kong]

On Sat, Apr 19, 2014 at 12:08:15PM +0800, Amos Kong wrote:
> 
> Hi all,
> 
> I'm working on this item in Upstream Networking todolist:
> 
> | - Sharing config interrupts
> |    Support more devices by sharing a single msi vector
> |    between multiple virtio devices.
> |    (Applies to virtio-blk too).
> 
> I have this solution here, and only have draft patches of Solution
> 1 & 2, let's discuss if solution 3 is feasible.
> 
> * We should not introduce perf regression in this change
> * little effect is acceptable because we are _sharing_ interrupt
> 
> Solution 1:
> ==========
> share one LSI interrupt for configuration change of all virtio devices.
> Draft patch: share_lsi_for_all_config.patch
> 
> Solution 2:
> ==========
> share one MSI interrupt for configuration change of all virtio devices.
> Draft patch: share_msix_for_all_config.patch
> 
> Solution 3:
> ==========
> dynamic adjust interrupt policy when device is added or removed
> Current:
> -------
> - try to allocate a MSI to device's config
> - try to allocate a MSI for each vq
> - share one MSI for all vqs of one device
> - share one LSI for config & vqs of one device
> 
> additional dynamic policies:
> ---------------------------
> - if there is no enough MSI, adjust interrupt allocation for returning
>   some MSI
>    - try to find a device that allocated one MSI for each vq, change
>      it to share one MSI for saving the MSI
>    - try to share one MSI for config of all virtio_pci devices
>    - try to share one LSI for config of all devices
> 
> - if device is removed, try to re-allocate freed MSI to existed
>   devices, if they aren't in best status (one MSI for config, one
>   MSI for each vq)
>    - if config of all devices is sharing one LSI, try to upgrade it to MSI
>    - if config of all devices is sharing one MSI, try to allocate one
>      MSI for configuration change of each device
>    - try to find a device that is sharing one MSI for all vqs, try to
>      allocate one MSI for each vq
> 
> 
> 
> BTW, I saw we still notify all vqs even VIRTIO_PCI_ISR_CONFIG bit of
> isr is set, is it necessary?

[Reply myself]

Quote from Virtio-spec:

| 2.4.3 Dealing With Configuration Changes
| Some virtio PCI devices can change the device configuration state, as reflected
| in the virtio header in the PCI configuration space. In this case:
| 
| 1. If MSI-X capability is disabled: an interrupt is delivered and the sec-
| ond highest bit is set in the ISR Status field to indicate that the driver
| should re-examine the configuration space.

| Note that a single interrupt can
| indicate both that one or more virtqueue has been used and that the con-
| figuration space has changed: even if the config bit is set, virtqueues must
| be scanned. <<<<

It seems current code is fine.

 
> diff --git a/drivers/virtio/virtio_pci.c b/drivers/virtio/virtio_pci.c
> index 101db3f..176aabc 100644
> --- a/drivers/virtio/virtio_pci.c
> +++ b/drivers/virtio/virtio_pci.c
> @@ -259,9 +259,9 @@ static irqreturn_t vp_interrupt(int irq, void
> *opaque)
>  
>         /* Configuration change?  Tell driver if it wants to know. */
>         if (isr & VIRTIO_PCI_ISR_CONFIG)
> -               vp_config_changed(irq, opaque);
> -
> -       return vp_vring_interrupt(irq, opaque);
> +               return vp_config_changed(irq, opaque);
> +       else
> +               return vp_vring_interrupt(irq, opaque);
>  }
>  
>  static void vp_free_vectors(struct virtio_device *vdev)
> 
> -- 
> 			Amos.