Re: [RFC] hw/nvme: Use irqfd to send interrupts

[Klaus Jensen <its@irrelevant.dk>]

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On Jul 21 09:29, Stefan Hajnoczi wrote:
> On Wed, Jul 20, 2022, 22:36 Jinhao Fan <fanjinhao21s@ict.ac.cn> wrote:
> 
> > Hi Stefan,
> >
> > Thanks for the detailed explanation!
> >
> > at 6:21 PM, Stefan Hajnoczi <stefanha@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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