linux-kernel - Re: [External Email] Re: [PATCH 1/2] virtio-mmio: Process vrings more proactively

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Date:   Mon, 22 Jul 2019 10:28:30 +0800
From:   Fam Zheng <zhengfeiran@...edance.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Fei Li <lifei.shirley@...edance.com>
Cc:     linux-kernel@...r.kernel.org, Jason Wang <jasowang@...hat.com>,
        Pawel Moll <pawel.moll@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>
Subject: Re: [External Email] Re: [PATCH 1/2] virtio-mmio: Process vrings more
 proactively


On 7/19/19 11:17 PM, Michael S. Tsirkin wrote:
> On Fri, Jul 19, 2019 at 09:31:34PM +0800, Fei Li wrote:
>> From: Fam Zheng <zhengfeiran@...edance.com>
>>
>> This allows the backend to _not_ trap mmio read of the status register
>> after injecting IRQ in the data path, which can improve the performance
>> significantly by avoiding a vmexit for each interrupt.
>>
>> More importantly it also makes it possible for Firecracker to hook up
>> virtio-mmio with vhost-net, in which case there isn't a way to implement
>> proper status register handling.
>>
>> For a complete backend that does set either INT_CONFIG bit or INT_VRING
>> bit upon generating irq, what happens hasn't changed.
>>
>> Signed-off-by: Fam Zheng <zhengfeiran@...edance.com>
> This has a side effect of skipping vring callbacks
> if they trigger at the same time with a config
> interrupt.
> I don't see why this is safe.

Good point! I think the block can be moved out from the else block and 
run unconditionally then.

Fam


>
>
>> ---
>>   drivers/virtio/virtio_mmio.c | 4 +---
>>   1 file changed, 1 insertion(+), 3 deletions(-)
>>
>> diff --git a/drivers/virtio/virtio_mmio.c b/drivers/virtio/virtio_mmio.c
>> index e09edb5c5e06..9b42502b2204 100644
>> --- a/drivers/virtio/virtio_mmio.c
>> +++ b/drivers/virtio/virtio_mmio.c
>> @@ -295,9 +295,7 @@ static irqreturn_t vm_interrupt(int irq, void *opaque)
>>   	if (unlikely(status & VIRTIO_MMIO_INT_CONFIG)) {
>>   		virtio_config_changed(&vm_dev->vdev);
>>   		ret = IRQ_HANDLED;
>> -	}
>> -
>> -	if (likely(status & VIRTIO_MMIO_INT_VRING)) {
>> +	} else {
>>   		spin_lock_irqsave(&vm_dev->lock, flags);
>>   		list_for_each_entry(info, &vm_dev->virtqueues, node)
>>   			ret |= vring_interrupt(irq, info->vq);
>> -- 
>> 2.11.0