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

[Jan Beulich <jbeulich@xxxxxxxx>]

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?

> And also we have another 2gb before bank[0].

Yes, but I talked about that before.

Jan