Re: [PATCH v1 11/27] xen/riscv: create APLIC DT node for guest domains

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.04.2026 10:43, Oleksii Kurochko wrote:
> On 4/1/26 5:16 PM, Jan Beulich wrote:
>> On 10.03.2026 18:08, Oleksii Kurochko wrote:
>>> +    res = fdt_property(fdt, "#interrupt-cells", data, len);
>>> +    if ( res )
>>> +        return res;
>>
>> So host properties are again directly handed through to the guest?
> 
> I am not sure that it can be different from what host has.
> 
>> Shouldn't the number of interrupts (aiui that's the "number of cells"
>> here) a guest gets be independent from the host it runs one?
> 
> The #interrupt-cells property specifies how many 32-bit values (cells) 
> are used to encode a single interrupt specifier when referencing this 
> interrupt controller from another device node.
> In this APLIC schema, it's fixed at const: 2, meaning every interrupt 
> reference requires exactly two cells — typically:
> 
> Cell 1 — the interrupt source number (which of the riscv,num-sources 
> wired inputs)
> Cell 2 — the interrupt type/trigger flags (e.g. edge vs. level, active 
> high/low)

So what if #interrupt-cells is 3 in the DT Xen is handed? If Xen can
cope, should that value really also be handed through to guests?

>>> +    regs = dt_get_property(aplic_node, "reg", &len);
>>> +    if ( !regs )
>>> +    {
>>> +        printk("%s: Can't find 'reg' property\n", aplic_node->full_name);
>>> +        return -FDT_ERR_XEN(ENOENT);
>>> +    }
>>> +
>>> +    res = fdt_property(fdt, "reg", regs, len);
>>> +    if ( res )
>>> +        return res;
>>> +
>>> +    data = dt_get_property(aplic_node, "riscv,num-sources", &len);
>>> +    if ( !data )
>>> +    {
>>> +        printk("%s: Can't find 'riscv,num-sources' property\n",
>>> +                aplic_node->full_name);
>>> +        return -FDT_ERR_XEN(ENOENT);
>>> +    }
>>> +
>>> +    res = fdt_property(fdt, "riscv,num-sources", data, len);
>>> +    if ( res )
>>> +        return res;
>>
>> Or maybe this is the number of interrupts?
> 
> This is the total count of hardware interrupt lines wired into this 
> APLIC domain.
> 
> It could be independent from the host it runs one but looking at the 
> possible range [1,1023] for this property if we will put for a guest 
> lets say 22 but new host support only 20 when we will be in a trouble 
> anyway.

Correct. But if you had some hosts with 1024 and some with 256, how
would you bring up a guest on the former to later be able to migrate
it to one of the latter when guests inherit the count from the host?

> It seems like it is another one thing where it will be needed to check 
> that new host has enough number of interrupts and then just deny 
> migration to such host.

Yes.

Jan