Re: [PATCH v6 03/20] xen/riscv: introduce extenstion support check by compiler

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.03.2024 20:44, Conor Dooley wrote:
> On Wed, Mar 20, 2024 at 07:58:05PM +0100, Oleksii wrote:
>> On Mon, 2024-03-18 at 17:58 +0100, Jan Beulich wrote:
>>> On 15.03.2024 19:05, Oleksii Kurochko wrote:
>>>> Currently, RISC-V requires two extensions: _zbb and _zihintpause.
>>>
>>> Do we really require Zbb already?
>> After an introduction of Andrew C. patches [1] it is requited for
>> __builtin_ffs{l}
>>
>> [1]
>> https://lore.kernel.org/xen-devel/20240313172716.2325427-1-andrew.cooper3@xxxxxxxxxx/T/#t
>>>
>>>> This patch introduces a compiler check to check if these extensions
>>>> are supported.
>>>> Additionally, it introduces the riscv/booting.txt file, which
>>>> contains
>>>> information about the extensions that should be supported by the
>>>> platform.
>>>>
>>>> In the future, a feature will be introduced to check whether an
>>>> extension
>>>> is supported at runtime.
>>>> However, this feature requires functionality for parsing device
>>>> tree
>>>> source (DTS), which is not yet available.
>>>
>>> Can't you query the CPU for its features?
>> I couldn't find such reg ( or SBI call ) in the spec.
> 
> There isn't.
> 
>> SBI call sbi_probe_extension() exists, but it doesn't check for every
>> possible extension. As far as I understand it checks only for that one
>> which are present in SBI spec.
> 
> Yeah, it only checks for SBI extensions, not ISA extensions.

And there was never a consideration to add, at the architecture level,
some straightforward way for all layers of software to be able to
easily detect availability of extensions? I find the lack thereof
pretty surprising ...

Jan