Re: [PATCH v1] xen/config.h: Move BITS_PER_* definitions from asm/config.h to xen/config.h

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 27/03/2025 8:21 am, Jan Beulich wrote:
> On 27.03.2025 01:44, Andrew Cooper wrote:
>> On 26/03/2025 5:47 pm, Oleksii Kurochko wrote:
>>> diff --git a/xen/include/xen/config.h b/xen/include/xen/config.h
>>> index d888b2314d..dbbf2fce62 100644
>>> --- a/xen/include/xen/config.h
>>> +++ b/xen/include/xen/config.h
>>> @@ -98,4 +98,13 @@
>>>  #define ZERO_BLOCK_PTR ((void *)-1L)
>>>  #endif
>>>  
>>> +#define BYTES_PER_LONG  __SIZEOF_LONG__
>>> +
>>> +#define BITS_PER_BYTE   __CHAR_BIT__
>>> +#define BITS_PER_INT    (__SIZEOF_INT__ << 3)
>>> +#define BITS_PER_LONG   (BYTES_PER_LONG << 3)
>>> +#define BITS_PER_LLONG  (__SIZEOF_LONG_LONG__ << 3)
>>> +
>>> +#define POINTER_ALIGN   __SIZEOF_POINTER__
>> See how much nicer this is.  This patch possibly wants to wait until
>> I've fixed the compiler checks to update to the new baseline, or we can
>> just say that staging is staging and corner case error messages are fine.
>>
>> One thing, you have to replace the "<< 3" as you're hard-coding 8 here
>> and ignoring __CHAR_BIT__.
>>
>> I'd suggest keeping the BITS_PER_BYTE on the LHS, e.g:
>>
>> #define BITS_PER_INT    (BITS_PER_BYTE * __SIZEOF_INT__)
>>
>> which tabulates better.
>>
>> I suggest keeping BITS_PER_XEN_ULONG to be arch-local.
> I agree here despite ...
>
>>   ARM is the
>> odd-one-out having a non-64bit arch use a 64bit XEN_ULONG.
> ... not agreeing here: x86 is the odd-one-out; I sincerely hope any new ports
> to 32-bit architectures / flavors will avoid compat layer translation by 
> making
> this type a proper 64-bit one. Architectures truly being 32-bit only, with no
> expectation of a 64-bit flavor ever appearing, would be different.

I have some very firm opinions about this.

It is an error that the type xen_ulong exists, anywhere.  The fact it
wasn't named guest_ulong shows a profound misunderstanding of its
purpose and usage in the API/ABI.

Similarly, BITS_PER_XEN_ULONG is buggily named, and should be
BITS_PER_GUEST_ULONG, as demonstrated by it's singular use in Xen
(calculating BITS_PER_EVTCHN_WORD(d)).

ARM declaring that arm32 uses 64-bit xen_ulongs was cutting a corner
that's going to bite twice as hard when 128bit comes along, and
RISCV-128 is in progress already.

All of this needs purging from the API/ABIs before RISC-V/PPC inherit
the mistakes that are holding x86 and ARM back.

~Andrew