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

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Date:   Thu, 25 Jul 2019 10:33:25 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Fam Zheng <zhengfeiran@...edance.com>
Cc:     Fei Li <lifei.shirley@...edance.com>, 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 Mon, Jul 22, 2019 at 10:28:30AM +0800, Fam Zheng wrote:
> 
> 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


Won't same callback run from multiple irq handlers then?
Running multiple vring callbacks at the same time isn't
a good idea either.

> 
> > 
> > 
> > > ---
> > >   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