On Jul 21 09:29, Stefan Hajnoczi wrote: > On Wed, Jul 20, 2022, 22:36 Jinhao Fan <fanjinhao...@ict.ac.cn> wrote: > > > Hi Stefan, > > > > Thanks for the detailed explanation! > > > > at 6:21 PM, Stefan Hajnoczi <stefa...@gmail.com> wrote: > > > > > Hi Jinhao, > > > Thanks for working on this! > > > > > > irqfd is not necessarily faster than KVM ioctl interrupt injection. > > > > > > There are at least two non performance reasons for irqfd: > > > 1. It avoids QEMU emulation code, which historically was not thread safe > > and needed the Big QEMU Lock. IOThreads don't hold the BQL and therefore > > cannot safely call the regular interrupt emulation code in QEMU. I think > > this is still true today although parts of the code may now be less reliant > > on the BQL. > > > > This probably means we need to move to irqfd when iothread support is added > > in qemu-nvme. > > > > Yes. You can audit the interrupt code but I'm pretty sure there is shared > state that needs to be protected by the BQL. So the NVMe emulation code > probably needs to use irqfd to avoid the interrupt emulation code. > > > > > 2. The eventfd interface decouples interrupt injection from the KVM > > ioctl interface. Vhost kernel and vhost-user device emulation code has no > > dependency on KVM thanks to irqfd. They work with any eventfd, including > > irqfd. > > > > This is contrary to our original belief. Klaus once pointed out that irqfd > > is KVM specific. I agreed with him since I found irqfd implementation is in > > virt/kvm/eventfd.c. But irqfd indeed avoids the KVM ioctl call. Could you > > elaborate on what “no dependency on KVM” means? > > > > "They work with any eventfd, including irqfd" > > If you look at the vhost kernel or vhost-user code, you'll see they just > signal the eventfd. It doesn't have to be an irqfd. > > An irqfd is a specific type of eventfd that the KVM kernel module > implements to inject interrupts when the eventfd is signaled. > > By the way, this not only decouples vhost from the KVM kernel module, but > also allows QEMU to emulate MSI-X masking via buffering the interrupt in > userspace. > >
The virtio dataplane (iothread support) only works with kvm if I am not mistaken, so I guess this is similar to what we want to do here. If we dont have KVM, we wont use iothread and we wont use the kvm irqchip/irqfd. Am I understanding this correctly?
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