Re: [PATCH 5/8] xen/evtchn: don't close the static event channel.

[Julien Grall <julien@xxxxxxx>]

(+ Juergen for the Linux question)

On 06/07/2022 11:42, Rahul Singh wrote:
> Hi Julien,
>
> > On 5 Jul 2022, at 2:56 pm, Julien Grall <julien@xxxxxxx> wrote:
> >
> >
> >
> > On 05/07/2022 14:28, Rahul Singh wrote:
> > > Hi Julien,
> > Hi Rahul,
> >
> > > > On 28 Jun 2022, at 4:18 pm, Julien Grall <julien@xxxxxxx> wrote:
> > > > > a new driver in linux kernel, etc where right now we just need to introduce an 
> > > > > extra IOCTL in linux to support this feature.
> > > > I don't understand why would need a new driver, etc. Given that you are introducing a new 
> > > > IOCTL you could pass a flag to say "This is a static event channel so don't close 
> > > > it".
> > > I tried to implement other solutions to this issue. We can introduce a new event channel 
> > > state “ECS_STATIC” and set the
> > > event channel state to ECS_STATIC when Xen allocate and create the static event 
> > > channels.
> >  From what you wrote, ECS_STATIC is just an interdomain behind but where you 
> > want Xen to prevent closing the port.
> >
> >  From Xen PoV, it is still not clear why this is a problem to let Linux closing 
> > such port. From the guest PoV, there are other way to pass this information 
> > (see below).
> If Linux closes the port, the static event channel created by Xen associated 
> with such port will not be available to use afterward.
>
> When I started implemented the static event channel series, I thought the 
> static event channel has to be available for use during
> the lifetime of the guest. This patch avoids closing the port if the Linux 
> user-space application wants to use the event channel again.
>
> This patch is fixing the problem for Linux OS, and I agree with you that we 
> should not modify the Xen to fix the Linux problem.
> Therefore, If the guest decided to close the static event channel, Xen will 
> close the port. Event Chanel associated with the port
> will not be available for use after that.I will discard this patch in the next 
> series.
>
> > >  From guest OS we can check if the event channel is static (via 
> > > EVTCHNOP_status()  hypercall ), if the event channel is
> > > static don’t try to close the event channel. If guest OS try to close the static event 
> > > channel Xen will return error as static event channel can’t be closed.
> > Why do you need this? You already need a binding indicating which ports will be 
> > pre-allocated. So you could update your binding to pass a flag telling Linux "don't 
> > close it".
> >
> > I have already proposed that before and I haven't seen any explanation why this 
> > is not a viable solution.
> Sorry I didn’t mention this earlier, I started with your suggestion to fix the 
> issue but after going through the Linux evtchn driver code
> it is not straight forward to tell Linux don’t close the port. Let me try to 
> explain.
>
> In Linux, struct user_evtchn {} is the struct that hold the information for 
> each user evtchn opened. We can add one bool parameter in this struct to tell 
> Linux driver
> via IOCTL if evtchn is static. When user application close the fd 
> "/dev/xen/evtchn” , evtchn_release() will traverse all the evtchn and call 
> evtchn_unbind_from_user()
> for each evtchn. evtchn_unbind_from_user() will call  __unbind_from_irq(irq) that will call 
> xen_evtchn_close() . We need references to "struct user_evtchn” in
> function __unbind_from_irq() to pass as argument to xen_evtchn_close() not to 
> close the static event channel.  I am not able to find any way to get
> struct user_evtchn in function __unbind_from_irq() , without modifying the 
> other Linux structure.
>
> Regards,
> Rahul
--
Julien Grall