Re: [PATCH v4 25/25] xen/riscv: add initial dom0less infrastructure support

[Jan Beulich <jbeulich@xxxxxxxx>]

On 01.07.2026 17:24, Oleksii Kurochko wrote:
> On 6/30/26 9:28 AM, Jan Beulich wrote:
>> On 26.06.2026 17:46, Oleksii Kurochko wrote:
>>> --- a/xen/arch/riscv/include/asm/guest-layout.h
>>> +++ b/xen/arch/riscv/include/asm/guest-layout.h
>>> @@ -32,4 +32,16 @@
>>>   #define GUEST_RAM_BANK_BASES   { GUEST_RAM0_BASE, GUEST_RAM1_BASE }
>>>   #define GUEST_RAM_BANK_SIZES   { GUEST_RAM0_SIZE, GUEST_RAM1_SIZE }
>>>   
>>> +/*
>>> + * The guest magic region holds Xen-reserved pages mapped into the guest's
>>> + * physical address space (shared info, grant table, etc.). The only real
>>> + * constraint is that the GUEST_MAGIC_SIZE-byte region must not overlap
>>> + * guest RAM (the GUEST_RAMx banks) or the emulated device regions defined
>>> + * above; the exact base is otherwise arbitrary. Here it is placed in the
>>> + * unused gap below GUEST_RAM0_BASE (0x80000000), but a hole after a RAM
>>> + * bank would work equally well.
>>> + */
>>> +#define GUEST_MAGIC_BASE  _UL(0x79000000)
>>> +#define GUEST_MAGIC_SIZE  _UL(0x01000000)
>>
>> ... while 16Mb may seem a lot, it feels pretty little for 64-bit guests.
>> Even in just Sv39 mode they have ample VA space to map a bigger region.
>> (As iirc indicated before, a static upper bound looks questionable to me
>> anyway.)
> 
> I think the comment should be updated as for grants will be separate region.
> 
> For all others, it looks like 16MB is more then enough. For example, Arm 
> has only 4 used pages (CONSOLE=0, XENSTORE=1, MEMACCESS=2, VUART=3).

Then the question goes the other way around: Why 16Mb? Won't e.g. 2Mb
suffice?

> So will you be okay with reworking of the comment to:
> /*
>   * The guest magic region holds the Xen-reserved pages mapped into the
>   * guest's physical address space. The only real constraint on
>   * GUEST_MAGIC_BASE/SIZE is that the region must not overlap guest RAM
>   * (the GUEST_RAMx banks) or the emulated device regions defined above;
>   * the exact base is otherwise arbitrary. Here it is placed in the 
> unused gap
>   * below GUEST_RAM0_BASE (0x80000000), but a hole after a RAM bank 
> would work
>   * equally well.
>   */
> 
> And add to the commit message that:
> ```
> A separate region for grant tables will be introduced at the same time 
> as the introduction of the grant table for RISC-V.
> ```

That's quite a bit better, yes.

Jan