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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 16 Jul 2020 02:13:00 -0400

On Thu, Jul 16, 2020 at 09:39:17AM +0800, 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?
> 
>     For 2) it looks to me guest should deal with the irq synchronization when
>     mask or unmask MSIX vectors.
> 
> Agreed!

Well we need to make sure during a switch each interrupt is reported
*somewhere*: either irq or eventfd - and not lost.

> Thanks,
> BR
> Zhu Lingshan
> 
> 
>     What's your thought?
> 
>     Thanks
> 
> 
> 
> 
> 
>         Zhu Lingshan (7):
>            vhost: introduce vhost_call_ctx
>            kvm/vfio: detect assigned device via irqbypass manager
>            vhost_vdpa: implement IRQ offloading functions in vhost_vdpa
>            vDPA: implement IRQ offloading helpers in vDPA core
>            virtio_vdpa: init IRQ offloading function pointers to NULL.
>            ifcvf: replace irq_request/free with helpers in vDPA core.
>            irqbypass: do not start consumer or producer when failed to connect
> 
>           arch/x86/kvm/x86.c              | 10 ++++--
>           drivers/vdpa/ifcvf/ifcvf_main.c | 11 +++---
>           drivers/vdpa/vdpa.c             | 46 +++++++++++++++++++++++++
>           drivers/vhost/Kconfig           |  1 +
>           drivers/vhost/vdpa.c            | 75
>         +++++++++++++++++++++++++++++++++++++++--
>           drivers/vhost/vhost.c           | 22 ++++++++----
>           drivers/vhost/vhost.h           |  9 ++++-
>           drivers/virtio/virtio_vdpa.c    |  2 ++
>           include/linux/vdpa.h            | 11 ++++++
>           virt/kvm/vfio.c                 |  2 --
>           virt/lib/irqbypass.c            | 16 +++++----
>           11 files changed, 181 insertions(+), 24 deletions(-)
> 
> 
> 
>