> On Jan 22, 2025, at 12:33 PM, Christian Borntraeger <borntraeger@xxxxxxxxxxxxx> wrote:
>
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>
> Am 22.01.25 um 15:44 schrieb Boyer, Andrew:
> [...]
>
>>>>> --- a/drivers/block/virtio_blk.c
>>>>> +++ b/drivers/block/virtio_blk.c
>>>>> @@ -379,14 +379,10 @@ static void virtio_commit_rqs(struct blk_mq_hw_ctx *hctx)
>>>>> {
>>>>> struct virtio_blk *vblk = hctx->queue->queuedata;
>>>>> struct virtio_blk_vq *vq = &vblk->vqs[hctx->queue_num];
>>>>> - bool kick;
>>>>> spin_lock_irq(&vq->lock);
>>>>> - kick = virtqueue_kick_prepare(vq->vq);
>>>>> + virtqueue_kick(vq->vq);
>>>>> spin_unlock_irq(&vq->lock);
>>>>> -
>>>>> - if (kick)
>>>>> - virtqueue_notify(vq->vq);
>>>>> }
>>>>
>>>> I would assume this will be a performance nightmare for normal IO.
>>>
>>
>> Hello Michael and Christian and Jason,
>> Thank you for taking a look.
>>
>> Is the performance concern that the vmexit might lead to the underlying virtual storage stack doing the work immediately? Any other job posting to the same queue would presumably be blocked on a vmexit when it goes to attempt its own notification. That would be almost the same as having the other job block on a lock during the operation, although I guess if you are skipping notifications somehow it would look different.
>
> The performance concern is that you hold a lock and then exit. Exits are expensive and can schedule so you will increase the lock holding time significantly. This is begging for lock holder preemption.
> Really, dont do it.
If holding a lock at all is unworkable, do you have a suggestion for how we might fix the correctness issue?
Because this is definitely a correctness issue.
I don't see anything in the spec about requiring support for out-of-order notifications.
-Andrew
