Re: [PATCH v1 27/27] xen/riscv: add initial dom0less infrastructure support

[Oleksii Kurochko <oleksii.kurochko@gmail.com>]

On 4/7/26 4:11 PM, Jan Beulich wrote:
> On 10.03.2026 18:09, Oleksii Kurochko wrote:
>> Enable dom0less support for RISC-V by selecting HAS_DOM0LESS and
>> providing the minimal architecture hooks required by the common
>> dom0less infrastructure.
>>
>> Add stub implementations for architecture-specific helpers used when
>> building domains from the device tree. These currently perform no
>> additional work but allow the generic dom0less code to build and run
>> on RISC-V.
>>
>> Introduce max_init_domid as a runtime variable rather than a constant
>> so that it can be updated during dom0less domain creation.
>>
>> Provide missing helpers and definitions required by the domain
>> construction code,
> 
> I'm wondering about the splitting among patches: There's half a dozen
> (effectively stub) functions which are added here, and then there is
> the single init_vuart() which was split out into the earlier patch.
> What's the pattern behind this, i.e. why isn't init_vuart() also
> being added here?

If it'll be more convenient I am okay to merge prev. patch with the 
current one.

init_vuart() is in a separate patch as it has some useful functionality 
inside and thereby it will be more convenient to review.

> 
>> including domain bitness helpers and the
>> p2m_set_allocation() prototype.
>>
>> Additionally define the guest magic memory region in the public
>> RISC-V interface.
>>
>> Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
>> ---
>> Open questions:
>>   - Shouldn't declaration/defintion of max_init_domid move to common code
>>     instead of having it for each architecture separately? If yes, then what
>>     would be the best place.
> 
> What would you use to decide whether the declaration or #define is
> needed? (Plausible headers to put it can surely be found: console.h,
> domain.h, and perhaps more.)

I thought about to wrap that with CONFIG_DOM0LESS_BOOT as the 
declaration is needed only for this case, for all other cases it is just 
#define.

Or as an option we could always use declaration all the time. It won't 
increase size of Xen too much or decrease performance because of 
variable access.

> 
>> --- a/xen/arch/riscv/include/asm/domain.h
>> +++ b/xen/arch/riscv/include/asm/domain.h
>> @@ -20,6 +20,14 @@ struct hvm_domain
>>       uint64_t              params[HVM_NR_PARAMS];
>>   };
>>   
>> +#ifdef CONFIG_RISCV_64
>> +#define is_32bit_domain(d) (0)
>> +#define is_64bit_domain(d) (1)
>> +#else
>> +#define is_32bit_domain(d) (1)
>> +#define is_64bit_domain(d) (0)
>> +#endif
> 
> First, please use true/false. Then, while I agree with the RV32 part, 32-bit
> guests surely will need to be an option on a 64-bit hypervisor. Imo you'd
> better introduced a field in struct arch_domain to carry that information
> (or to derive it from) right away. That wouldn't be set to non-zero for the
> time being, i.e. that same constant-true/false would still result.
> 
> Otherwise I don't see why you use #ifdef; you could then have things
> simpler as
> 
> #define is_32bit_domain(d) IS_ENABLED(CONFIG_RISCV_32)
> #define is_64bit_domain(d) IS_ENABLED(CONFIG_RISCV_64)
> 
> (but I specifically don't recommend going this route).

I will introduce a type in struct arch_domain then.

> 
>> --- a/xen/include/public/arch-riscv.h
>> +++ b/xen/include/public/arch-riscv.h
>> @@ -58,6 +58,9 @@ typedef uint64_t xen_ulong_t;
>>   #define GUEST_RAM_BANK_BASES   { GUEST_RAM0_BASE }
>>   #define GUEST_RAM_BANK_SIZES   { GUEST_RAM0_SIZE }
>>   
>> +#define GUEST_MAGIC_BASE  xen_mk_ullong(0x39000000)
>> +#define GUEST_MAGIC_SIZE  xen_mk_ullong(0x01000000)
> 
> What is this, and why does it need putting in the public interface? 

In other patch series such related things will live in 
asm/guest-layout.h. It will be moved there after rebase on top of that 
patch series.

It is needed now only for common dom0less code compilation as at the 
moment we don't use xenstore pages for dom0less.

But generally it is region which is used to allocate "magic pages" 
-special pages that facilitate communication between the guest and the 
Xen hypervisor such as Console, XenStore pages etc.

I have in my TODO to understand how to remove requirement to have this 
fixed region from dom0less common code.

Plus
> how come the numbers are exactly the same as what Arm uses?

It could be different. But this region is free for RISC-V too so it is 
fine to re-use.

~ Oleksii