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SubjectRe: virtio-blk: should num_vqs be limited by num_possible_cpus()?
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On 3/13/19 1:33 AM, Cornelia Huck wrote:
> On Tue, 12 Mar 2019 10:22:46 -0700 (PDT)
> Dongli Zhang <dongli.zhang@oracle.com> wrote:
>
>> I observed that there is one msix vector for config and one shared vector
>> for all queues in below qemu cmdline, when the num-queues for virtio-blk
>> is more than the number of possible cpus:
>>
>> qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=6"
>>
>> # cat /proc/interrupts
>> CPU0 CPU1 CPU2 CPU3
>> ... ...
>> 24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config
>> 25: 0 0 0 59 PCI-MSI 65537-edge virtio0-virtqueues
>> ... ...
>>
>>
>> However, when num-queues is the same as number of possible cpus:
>>
>> qemu: "-smp 4" while "-device virtio-blk-pci,drive=drive-0,id=virtblk0,num-queues=4"
>>
>> # cat /proc/interrupts
>> CPU0 CPU1 CPU2 CPU3
>> ... ...
>> 24: 0 0 0 0 PCI-MSI 65536-edge virtio0-config
>> 25: 2 0 0 0 PCI-MSI 65537-edge virtio0-req.0
>> 26: 0 35 0 0 PCI-MSI 65538-edge virtio0-req.1
>> 27: 0 0 32 0 PCI-MSI 65539-edge virtio0-req.2
>> 28: 0 0 0 0 PCI-MSI 65540-edge virtio0-req.3
>> ... ...
>>
>> In above case, there is one msix vector per queue.
>
> Please note that this is pci-specific...
>
>>
>>
>> This is because the max number of queues is not limited by the number of
>> possible cpus.
>>
>> By default, nvme (regardless about write_queues and poll_queues) and
>> xen-blkfront limit the number of queues with num_possible_cpus().
>
> ...and these are probably pci-specific as well.

Not pci-specific, but per-cpu as well.

>
>>
>>
>> Is this by design on purpose, or can we fix with below?
>>
>>
>> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
>> index 4bc083b..df95ce3 100644
>> --- a/drivers/block/virtio_blk.c
>> +++ b/drivers/block/virtio_blk.c
>> @@ -513,6 +513,8 @@ static int init_vq(struct virtio_blk *vblk)
>> if (err)
>> num_vqs = 1;
>>
>> + num_vqs = min(num_possible_cpus(), num_vqs);
>> +
>> vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
>> if (!vblk->vqs)
>> return -ENOMEM;
>
> virtio-blk, however, is not pci-specific.
>
> If we are using the ccw transport on s390, a completely different
> interrupt mechanism is in use ('floating' interrupts, which are not
> per-cpu). A check like that should therefore not go into the generic
> driver.
>

So far there seems two options.

The 1st option is to ask the qemu user to always specify "-num-queues" with the
same number of vcpus when running x86 guest with pci for virtio-blk or
virtio-scsi, in order to assign a vector for each queue.

Or, is it fine for virtio folks to add a new hook to 'struct virtio_config_ops'
so that different platforms (e.g., pci or ccw) would use different ways to limit
the max number of queues in guest, with something like below?

So far both virtio-scsi and virtio-blk would benefit from the new hook.

---
drivers/block/virtio_blk.c | 2 ++
drivers/virtio/virtio_pci_common.c | 6 ++++++
drivers/virtio/virtio_pci_common.h | 2 ++
drivers/virtio/virtio_pci_legacy.c | 1 +
drivers/virtio/virtio_pci_modern.c | 2 ++
include/linux/virtio_config.h | 11 +++++++++++
6 files changed, 24 insertions(+)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 4bc083b..93cfeda 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -513,6 +513,8 @@ static int init_vq(struct virtio_blk *vblk)
if (err)
num_vqs = 1;

+ num_vqs = virtio_calc_num_vqs(vdev, num_vqs);
+
vblk->vqs = kmalloc_array(num_vqs, sizeof(*vblk->vqs), GFP_KERNEL);
if (!vblk->vqs)
return -ENOMEM;
diff --git a/drivers/virtio/virtio_pci_common.c b/drivers/virtio/virtio_pci_common.c
index d0584c0..ce021d1 100644
--- a/drivers/virtio/virtio_pci_common.c
+++ b/drivers/virtio/virtio_pci_common.c
@@ -409,6 +409,12 @@ int vp_find_vqs(struct virtio_device *vdev, unsigned nvqs,
return vp_find_vqs_intx(vdev, nvqs, vqs, callbacks, names, ctx);
}

+/* the config->calc_num_vqs() implementation */
+unsigned short vp_calc_num_vqs(unsigned short num_vqs)
+{
+ return min_t(unsigned short, num_possible_cpus(), num_vqs);
+}
+
const char *vp_bus_name(struct virtio_device *vdev)
{
struct virtio_pci_device *vp_dev = to_vp_device(vdev);
diff --git a/drivers/virtio/virtio_pci_common.h b/drivers/virtio/virtio_pci_common.h
index 0227100..cc5ac80 100644
--- a/drivers/virtio/virtio_pci_common.h
+++ b/drivers/virtio/virtio_pci_common.h
@@ -134,6 +134,8 @@ int vp_find_vqs(struct virtio_device *vdev, unsigned nvqs,
struct virtqueue *vqs[], vq_callback_t *callbacks[],
const char * const names[], const bool *ctx,
struct irq_affinity *desc);
+/* the config->calc_num_vqs() implementation */
+unsigned short vp_calc_num_vqs(unsigned short num_vqs);
const char *vp_bus_name(struct virtio_device *vdev);

/* Setup the affinity for a virtqueue:
diff --git a/drivers/virtio/virtio_pci_legacy.c b/drivers/virtio/virtio_pci_legacy.c
index eff9ddc..69d1050 100644
--- a/drivers/virtio/virtio_pci_legacy.c
+++ b/drivers/virtio/virtio_pci_legacy.c
@@ -209,6 +209,7 @@ static const struct virtio_config_ops virtio_pci_config_ops = {
.bus_name = vp_bus_name,
.set_vq_affinity = vp_set_vq_affinity,
.get_vq_affinity = vp_get_vq_affinity,
+ .calc_num_vqs = vp_calc_num_vqs,
};

/* the PCI probing function */
diff --git a/drivers/virtio/virtio_pci_modern.c b/drivers/virtio/virtio_pci_modern.c
index 07571da..f04e44a 100644
--- a/drivers/virtio/virtio_pci_modern.c
+++ b/drivers/virtio/virtio_pci_modern.c
@@ -460,6 +460,7 @@ static const struct virtio_config_ops
virtio_pci_config_nodev_ops = {
.bus_name = vp_bus_name,
.set_vq_affinity = vp_set_vq_affinity,
.get_vq_affinity = vp_get_vq_affinity,
+ .calc_num_vqs = vp_calc_num_vqs,
};

static const struct virtio_config_ops virtio_pci_config_ops = {
@@ -476,6 +477,7 @@ static const struct virtio_config_ops virtio_pci_config_ops = {
.bus_name = vp_bus_name,
.set_vq_affinity = vp_set_vq_affinity,
.get_vq_affinity = vp_get_vq_affinity,
+ .calc_num_vqs = vp_calc_num_vqs,
};

/**
diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
index bb4cc49..f32368b 100644
--- a/include/linux/virtio_config.h
+++ b/include/linux/virtio_config.h
@@ -65,6 +65,7 @@ struct irq_affinity;
* the caller can then copy.
* @set_vq_affinity: set the affinity for a virtqueue (optional).
* @get_vq_affinity: get the affinity for a virtqueue (optional).
+ * @calc_num_vqs: calculate the number of virtqueue (optional)
*/
typedef void vq_callback_t(struct virtqueue *);
struct virtio_config_ops {
@@ -88,6 +89,7 @@ struct virtio_config_ops {
const struct cpumask *cpu_mask);
const struct cpumask *(*get_vq_affinity)(struct virtio_device *vdev,
int index);
+ unsigned short (*calc_num_vqs)(unsigned short num_vqs);
};

/* If driver didn't advertise the feature, it will never appear. */
@@ -207,6 +209,15 @@ int virtio_find_vqs_ctx(struct virtio_device *vdev,
unsigned nvqs,
desc);
}

+static inline
+unsigned short virtio_calc_num_vqs(struct virtio_device *vdev,
+ unsigned short num_vqs)
+{
+ if (vdev->config->calc_num_vqs)
+ return vdev->config->calc_num_vqs(num_vqs);
+ return num_vqs;
+}
+
/**
* virtio_device_ready - enable vq use in probe function
* @vdev: the device
--
2.7.4

Thank you very much!

Dongli Zhang

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