Re: About Arm guests accessing to GICD_ICPENR

[Julien Grall <julien@xxxxxxx>]

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?

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

--
Julien Grall