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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Jan