Re: [PATCH v1 10/27] xen/riscv: generate IMSIC DT node for guest domains

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.04.2026 16:01, Oleksii Kurochko wrote:
> 
> 
> On 4/17/26 3:50 PM, Jan Beulich wrote:
>> On 17.04.2026 10:10, Oleksii Kurochko wrote:
>>> On 4/16/26 1:42 PM, Jan Beulich wrote:
>>>> On 10.04.2026 17:40, Oleksii Kurochko wrote:
>>>>> On 4/1/26 5:05 PM, Jan Beulich wrote:
>>>>>> On 10.03.2026 18:08, Oleksii Kurochko wrote:
>>>>>>> Guests using the IMSIC interrupt controller require a corresponding
>>>>>>> Device Tree description. Add support for generating an IMSIC node when
>>>>>>> building the guest DT.
>>>>>>>
>>>>>>> Keep a reference to the host IMSIC DT node and reuse its compatible
>>>>>>> property while constructing the guest-visible node.
>>>>>>
>>>>>> Again raises a migration concern. Presumably a guest would then be able
>>>>>> to migrate only to other hosts with the same compatible property.
>>>>>
>>>>> Right, but I don't think we can do too much and it is the simplest
>>>>> approach just to migrate to hosts with the same compatible property.
>>>>>
>>>>> If you concern is about DTS property then for IMSIC it will be always
>>>>> riscv,imsics according to the RISC-V DT spec. (IIRC, the only other
>>>>> option could be qemu,riscv). Actually, I can just hard code
>>>>> "riscv,imsic" explicitly instead of re-using of host name.
>>>>>
>>>>> If your concern is that we will migrate to a host which doesn't support
>>>>> IMSIC at all then we should or (a) don't migrate to such host or (b)
>>>>> provide an emulation of IMSIC. And option (b) would be terrible from at
>>>>> least performance point of view.
>>>>
>>>> That would all be only a secondary concern - in an IMSIC is needed right
>>>> now, so be it. My primary concern is with inheriting the host IMSIC's
>>>> properties.
>>> Could we do really something better?
>>>
>>> At the moment, the following properties are inhereted:
>>> node name -> not an issue at all, it is just a name of the node and
>>> generally doesn't matter what it is in the matter of DTB generation as
>>> phandle number of this node will be used as pointer to this node, not
>>> the name. If it would be better I can hard code just "imsic".
>>>
>>> compatible -> also, not a big issue. According to current DT RISC-V
>>> bindings it could be just "riscv,imsic". But then I have the question
>>> what if one day someone will come up with own implementation of IMSIC
>>> then "riscv,own-imsic" will be in host DTB and so I expect that guest
>>> DTB should have it as this h/w expects to use specifically OWN-IMSIC
>>> driver. So it seems okay to copy compatible from host DTB. Yes, if
>>> migration will happen where just "riscv,imsic" is supported then it
>>> can't be easily migrated to such h/w and its okay. But generally I
>>> expect that compatible will always riscv,imsic.
>>>
>>> riscv,num-ids -> generally, it could be any number from [63, 2047], but
>>> it seems like there is no any sense if put a value bigger then
>>> guest-num-ids=min(riscv,num-ids, riscv,num-guest-ids) for guest as h/w
>>> can't support more then that. At the same time I don't see too much
>>> sense to tell a guest that it supports less then guest-num-ids. What is
>>> the problem to give a guest a maximum that IMSIC could provide? With
>>> migration, of course, it will be an issue if new host will support
>>> different number of riscv,num-ids but I don't know what we can do better
>>> then just avoid migration to such hosts if old-num-ids > new-num-ids
>>> without significant performance drop.
>>
>> This is exactly why an admin may want to limit what a guest gets to
>> see / use.
> 
> Would it be better then provide a separate define for riscv, num-ids and 
> use it here? Maybe here it makes sense to re-use already provided by DT 
> binding property:
>    riscv,num-guest-ids:
>      $ref: /schemas/types.yaml#/definitions/uint32
>      minimum: 63
>      maximum: 2047
>      description:
>        Number of interrupt identities are supported by IMSIC guest 
> interrupt
>        file. When not specified it is assumed to be same as specified by the
>        riscv,num-ids property.
> And if riscv,num-guest-ids the use some predefined in define value?
> 
> Does it make sense then provide the similar defines for compatible and 
> node name or for that properties it would be okay to re-use what host 
> DTB IMSIC node provides?

I fear I can't answer this. I'm not knowledgeable enough on DT. To me,
guest properties are (originally) specified by a guest config file.
How that maps to DT (in particular in the dom0less case) I don't really
know.

Jan