RE: About Arm guests accessing to GICD_ICPENR

[Hongda Deng <Hongda.Deng@xxxxxxx>]

Hi Julien,

> On 06/09/2021 10:04, Hongda Deng wrote:
> > Hi Julien,
> 
> Hi Hongda,
> 
> > Xen provides vGIC to support Xen guests, and currently xen will return IO
> > unhandled when guests access GICD ICPENRn registers. This works fine with
> Linux
> > guests, for Linux won't access these registers. But for Zephyr, this 
> > mechanism
> > will cause IO dataabort on Zephyr's initialization which makes Zephyr get in
> > fatal error.
> >
> > One method to ease this is to let vGIC ignore GICD ICPENRn registers 
> > access. I
> > tested it with Linux guests and Zephyr guests, and both works fine. And I 
> > found
> > in this patch[1] that it would be more complex to touch the read part for
> > I{S,C}PENDR and the write part for ICPENDR,
> 
> Read to I{S,C}PENDR should already return. AFAIK, what's left
> unimplemented is write to ICPENDR.
> 
> > so could we ignore GICD ICPENDRn
> > registers access to ease Zephyr's initialization problem?
> Would you be able to provide more information on how Zephyr is using it?

Zephyr will try to clear pending state at its initialization, and the code can 
be
found here[1].

> 
> >
> > If Xen wants a complete GICD ICPENDRn emulation to fix it, do you have any
> > suggestions.
> 
> Emulating ICPENDR is not easy with the existing vGIC. It would be great
> to finally have a vGIC spec compliant, but I also appreciate this is
> going to take some time.

Yes, I agree with that.

> 
> Ignoring the access is probably the best we can do. However, this is
> also a risky approach because Zephyr (or another OS) may genuinely want
> to clear an already pending interrupt. So I would suggest to walk the
> interrupts that are meant to be modified and check whether they are
> already pending. If they are then print a message.

OK, I would add this check and warning message to my current patch to fix it 
in current stage. Is it OK?

> 
> This would make clear to the developper that something may go wrong
> later on.
> 
> Cheers,
> 
> --
> Julien Grall

[1] https://github.com/zephyrproject-rtos/zephyr/blob/eaf6cf745df3807e6e
cc941c3a30de6c179ae359/drivers/interrupt_controller/intc_gicv3.c#L274

Cheers
Hongda