On 2014/10/26 19:56, Michael S. Tsirkin wrote:
> On Sat, Oct 25, 2014 at 04:24:54PM +0800, john.liuli wrote:
>> From: Li Liu <john.liuli@xxxxxxxxxx>
>>
>> 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@xxxxxxxxxx>
>> ---
>> 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?
>
good catch, a stupid mistake.
>> +
>> + 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.
Yeah, AFAIK it's impossible to distribute works to different CPUs with
only one irq without MSI-X kind mechanism. Assign multiple gsis to one
device, obviously it's consumptive and not scalable. Any ideas? Thx.
>
>> --
>> 1.7.9.5
>>
>
> .
>
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