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Date:   Fri, 19 Jul 2019 11:17:00 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Fei Li <lifei.shirley@bytedance.com>
Cc:     linux-kernel@vger.kernel.org, Jason Wang <jasowang@redhat.com>,
        Pawel Moll <pawel.moll@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Fam Zheng <zhengfeiran@bytedance.com>
Subject: Re: [PATCH 1/2] virtio-mmio: Process vrings more proactively
Message-ID: <20190719111440-mutt-send-email-mst@kernel.org>
References: <20190719133135.32418-1-lifei.shirley@bytedance.com>
 <20190719133135.32418-2-lifei.shirley@bytedance.com>
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On Fri, Jul 19, 2019 at 09:31:34PM +0800, Fei Li wrote:
> From: Fam Zheng <zhengfeiran@bytedance.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@bytedance.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.


> ---
>  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