Re: [PATCH v2 10/11] xen/riscv: add definition of guest RAM banks

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

On 4/1/26 5:10 PM, Jan Beulich wrote:
> On 01.04.2026 16:53, Oleksii Kurochko wrote:
>>
>>
>> On 4/1/26 4:22 PM, Jan Beulich wrote:
>>> On 01.04.2026 15:57, Oleksii Kurochko wrote:
>>>> On 4/1/26 8:17 AM, Jan Beulich wrote:
>>>>> On 31.03.2026 18:14, Oleksii Kurochko wrote:
>>>>>> On 3/30/26 5:51 PM, Jan Beulich wrote:
>>>>>>> On 23.03.2026 17:29, Oleksii Kurochko wrote:
>>>>>>>> The dom0less solution uses defined RAM banks as compile-time constants,
>>>>>>>> so introduce macros to describe guest RAM banks.
>>>>>>>>
>>>>>>>> The reason for 2 banks is that there is typically always a use case for
>>>>>>>> low memory under 4 GB, but the bank under 4 GB ends up being small because
>>>>>>>> there are other things under 4 GB it can conflict with (interrupt
>>>>>>>> controller, PCI BARs, etc.).
>>>>>>> Fixed layouts like the one you suggest come with (potentially severe)
>>>>>>> downsides. For example, what if more than 2Gb of MMIO space are needed
>>>>>>> for non-64-bit BARs?
>>>>>> It looks where usually RAM on RISC-V boards start, so I expect that 2gb
>>>>>> before RAM start is enough for MMIO space.
>>>>> Likely in the common case. Board designers aren't constrained by this,
>>>>> though (aiui). Whereas you set in stone a single, fixed layout.
>>>>>
>>>>> Arm maintainers - since a similar fixed layout is used there iirc,
>>>>> could you chime in here, please?
>>>>>
>>>>>> Answering your question it will be an issue or it will also use some
>>>>>> space before banks, no?
>>>>> I fear I don't understand what you're trying to tell me.
>>>> I meant that there is also some space between banks and pretty big which
>>>> could be used for MMIO which could be used for non-64-bit BARs.
>>> I don't follow: Bank 0 extends to 4G. There's no space above it, below
>>> bank 1, which could be use for non-64-bit BARs.
>>
>> So we have two banks:
>> bank[0] -> [0x80000000, 0x100000000)
>> bank[1] -> [0x0200000000, 10000000000)
>>
>> So i think we have some space between them [0x100000000, 0x0200000000)
>> -> 4gb to be used for non-64-bit BARs.
> 
> But a non-64-bit BAR need to be assigned an address below 0x100000000?

Right, I had in mind that RV32 uses for guest Sv32x4 which could 
translate 34-bit GPA into 34-bit MPA and automatically applied that to 
32-bit BAR...

I can keep first 4gb for MMIO purpose and start bank[0] at 4gb as 34 MPA 
address space is more then enough to cover reserved 2gb of bank[0] after 
4gb.

~ Oleksii