Re: [PATCH v1 6/6] xen/riscv: enable DOMAIN_BUILD_HELPERS

[Jan Beulich <jbeulich@xxxxxxxx>]

On 18.02.2026 11:39, Oleksii Kurochko wrote:
> 
> On 2/13/26 2:11 PM, Jan Beulich wrote:
>> On 13.02.2026 13:54, Oleksii Kurochko wrote:
>>> On 2/12/26 5:39 PM, Jan Beulich wrote:
>>>> On 12.02.2026 17:21, Oleksii Kurochko wrote:
>>>>> --- a/xen/include/public/arch-riscv.h
>>>>> +++ b/xen/include/public/arch-riscv.h
>>>>> @@ -50,6 +50,14 @@ typedef uint64_t xen_ulong_t;
>>>>>    
>>>>>    #if defined(__XEN__) || defined(__XEN_TOOLS__)
>>>>>    
>>>>> +#define GUEST_RAM_BANKS   1
>>>>> +
>>>>> +#define GUEST_RAM0_BASE   xen_mk_ullong(0x80000000) /* 2GB of low RAM @ 
>>>>> 2GB */
>>>>> +#define GUEST_RAM0_SIZE   xen_mk_ullong(0x80000000)
>>>>> +
>>>>> +#define GUEST_RAM_BANK_BASES   { GUEST_RAM0_BASE }
>>>>> +#define GUEST_RAM_BANK_SIZES   { GUEST_RAM0_SIZE }
>>>> Hmm, does RISC-V really want to go with compile-time constants here?
>>> It is needed for allocate_memory() for guest domains, so it is expected
>>> to be compile-time constant with the current code of common dom0less
>>> approach.
>>>
>>> It represents the start of RAM address for DomU and the maximum RAM size
>>> (the actual size will be calculated based on what is mentioned in DomU node
>>> in dts) and then will be used to generate memory node for DomU 
>>> (GUEST_RAM0_BASE
>>> as RAM start address and min(GUEST_RAM0_SIZE, dts->domU->memory->size) as a
>>> RAM size).
>>>
>>>>    And
>>>> if so, why would guests be limited to just 2 Gb?
>>> It is enough for guest domain I am using in dom0less mode.
>> And what others may want to use RISC-V for once it actually becomes usable
>> isn't relevant? As you start adding things to the public headers, you will
>> need to understand that you can't change easily what once was put there.
>> Everything there is part of the ABI, and the ABI needs to remain stable
>> (within certain limits).
> 
> Considering this ...
> 
>>>> That may more efficiently
>>>> be RV32 guests then, with perhaps just an RV32 hypervisor.
>>> I  didn't get this point. Could you please explain differently what do you
>>> mean?
>> If all you want are 2Gb guests, why would such guests be 64-bit? And with
>> (iirc) RV32 permitting more than 4Gb (via PPN being 22 bits wide), perhaps
>> even a 32-bit hypervisor would suffice?
> 
> ... now I can agree that Xen should permit bigger amount of RAM. At least,
> (2^34-1) should be allowed for RV32 and so for RV64 so it could be used
> like a base for both of them. As RV64 allows (2^56 - 1) it makes sense
> to add another bank to cover range from 2^34 to (2^56 -1) for RV64 (and ifdef
> this second bank for  RV64).
> 
> Would it be better?

Having a 2nd bank right away for RV64 would seem better to me, yes. Whether
that means going all the way up to 2^56 I don't know.

In whether a public header can be changed, it also matters whether these
#define-s actually are meant to be exposed to guests (vs. merely the tool
stack). Longer-term, however, this is going to change (as we intend to move
to a fully stable ABI).

Jan