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 > > > > > >
