Subject: Re: [PATCH 0/7] *** IRQ offloading for vDPA ***

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 16 Jul 2020 02:15:24 -0400

On Thu, Jul 16, 2020 at 12:20:09PM +0800, Jason Wang wrote:
> 
> On 2020/7/16 下午12:13, Zhu, Lingshan wrote:
> > 
> > 
> > On 7/16/2020 12:02 PM, Jason Wang wrote:
> > > 
> > > On 2020/7/16 上午11:59, Zhu, Lingshan wrote:
> > > > 
> > > > On 7/16/2020 10:59 AM, Jason Wang wrote:
> > > > > 
> > > > > On 2020/7/16 上午9:39, Zhu, Lingshan wrote:
> > > > > > 
> > > > > > 
> > > > > > On 7/15/2020 9:43 PM, Jason Wang wrote:
> > > > > > > 
> > > > > > > On 2020/7/12 下午10:52, Zhu Lingshan wrote:
> > > > > > > > Hi All,
> > > > > > > > 
> > > > > > > > This series intends to implement IRQ offloading for
> > > > > > > > vhost_vdpa.
> > > > > > > > 
> > > > > > > > By the feat of irq forwarding facilities like posted
> > > > > > > > interrupt on X86, irq bypass can  help deliver
> > > > > > > > interrupts to vCPU directly.
> > > > > > > > 
> > > > > > > > vDPA devices have dedicated hardware backends like VFIO
> > > > > > > > pass-throughed devices. So it would be possible to setup
> > > > > > > > irq offloading(irq bypass) for vDPA devices and gain
> > > > > > > > performance improvements.
> > > > > > > > 
> > > > > > > > In my testing, with this feature, we can save 0.1ms
> > > > > > > > in a ping between two VFs on average.
> > > > > > > 
> > > > > > > 
> > > > > > > Hi Lingshan:
> > > > > > > 
> > > > > > > During the virtio-networking meeting, Michael spots
> > > > > > > two possible issues:
> > > > > > > 
> > > > > > > 1) do we need an new uAPI to stop the irq offloading?
> > > > > > > 2) can interrupt lost during the eventfd ctx?
> > > > > > > 
> > > > > > > For 1) I think we probably not, we can allocate an
> > > > > > > independent eventfd which does not map to MSIX. So
> > > > > > > the consumer can't match the producer and we
> > > > > > > fallback to eventfd based irq.
> > > > > > Hi Jason,
> > > > > > 
> > > > > > I wonder why we need to stop irq offloading, but if we
> > > > > > need to do so, maybe a new uAPI would be more intuitive
> > > > > > to me,
> > > > > > but why and who(user? qemu?) shall initialize this
> > > > > > process, based on what kinda of basis to make the
> > > > > > decision?
> > > > > 
> > > > > 
> > > > > The reason is we may want to fallback to software datapath
> > > > > for some reason (e.g software assisted live migration). In
> > > > > this case we need intercept device write to used ring so we
> > > > > can not offloading virtqueue interrupt in this case.
> > > > so add a VHOST_VDPA_STOP_IRQ_OFFLOADING? Then do we need a
> > > > VHOST_VDPA_START_IRQ_OFFLOADING, then let userspace fully
> > > > control this? Or any better approaches?
> > > 
> > > 
> > > Probably not, it's as simple as allocating another eventfd (but not
> > > irqfd), and pass it to vhost-vdpa. Then the offloading is disabled
> > > since it doesn't have a consumer.
> > OK, sounds like QEMU work, no need to take care in this series, right?
> 
> 
> That's my understanding.
> 
> Thanks

Let's confirm a switch happens atomically so each interrupt
is sent either to eventfd to guest directly though.

> > Thanks,
> > BR
> > Zhu Lingshan
> > > Thanks