Re: [PATCH v4 3/8] bits: introduce fixed-type genmasks

[Anshuman Khandual]

On 3/19/25 09:04, Anshuman Khandual wrote:
> On 3/19/25 07:16, Yury Norov wrote:
>> + Catalin Marinas, ARM maillist
>>
>> Hi Catalin and everyone,
> 
> Hello Yury,
> 
>>
>> Anshuman Khandual asked me to merge GENMASK_U128() saying it's
>> important for ARM to stabilize API. While it's a dead code, I
>> accepted his patch as he promised to add users shortly.
>>
>> Now it's more than half a year since that. There's no users,
>> and no feedback from Anshuman.
> 
> My apologies to have missed your email earlier. Please find response
> for the earlier email below as well.
> 
>>
>> Can you please tell if you still need the macro? I don't want to
>> undercut your development, but if you don't need 128-bit genmasks
>> there's no reason to have a dead code in the uapi.
> 
> The code base specifically using GENMASK_U128() has not been posted
> upstream (probably in next couple of months or so) till now, except
> the following patch which has been not been merged and still under
> review and development.
> 
> https://lore.kernel.org/lkml/20240801054436.612024-1-anshuman.khandual@xxxxxxx/
> 
>>
>> Thanks,
>> Yury
>>
>> On Wed, Mar 05, 2025 at 10:22:47AM -0500, Yury Norov wrote:
>>> + Anshuman Khandual <anshuman.khandual@xxxxxxx>
>>>
>>> Anshuman,
>>>
>>> I merged your GENMASK_U128() because you said it's important for your
>>> projects, and that it will get used in the kernel soon.
>>>
>>> Now it's in the kernel for more than 6 month, but no users were added.
>>> Can you clarify if you still need it, and if so why it's not used?
> 
> We would need it but although the code using GENMASK_U128() has not been
> posted upstream.
> 
>>>
>>> As you see, people add another fixed-types GENMASK() macros, and their
>>> implementation differ from GENMASK_U128().
> 
> I will take a look. Is GENMASK_U128() being problematic for the this new
> scheme ?
> 
>>>
>>> My second concern is that __GENMASK_U128() is declared in uapi, while
>>> the general understanding for other fixed-type genmasks is that they
>>> are not exported to users. Do you need this macro to be exported to
>>> userspace? Can you show how and where it is used there?
> 
> No, not atleast right now.
> 
> These were moved into uapi subsequently via the following commit.
> 
> 21a3a3d015aee ("tools headers: Synchronize {uapi/}linux/bits.h with the kernel sources")
> 
> But in general GENMASK_U128() is needed for generating 128 bit page table
> entries, related flags and masks whether in kernel or in user space for
> writing kernel test cases etc.

In the commit 947697c6f0f7 ("uapi: Define GENMASK_U128"), GENMASK_U128() gets defined
using __GENMASK_U128() which in turn calls __BIT128() - both of which are defined in
UAPI headers inside (include/uapi/linux/). 

Just wondering - are you suggesting to move these helpers from include/uapi/linux/ to
include/linux/bits.h instead ?

> 
>>>
>>> Thanks,
>>> Yury
>>>
>>>
>>> On Wed, Mar 05, 2025 at 10:00:15PM +0900, Vincent Mailhol via B4 Relay wrote:
>>>> From: Yury Norov <yury.norov@xxxxxxxxx>
>>>>
>>>> Add __GENMASK_t() which generalizes __GENMASK() to support different
>>>> types, and implement fixed-types versions of GENMASK() based on it.
>>>> The fixed-type version allows more strict checks to the min/max values
>>>> accepted, which is useful for defining registers like implemented by
>>>> i915 and xe drivers with their REG_GENMASK*() macros.
>>>>
>>>> The strict checks rely on shift-count-overflow compiler check to fail
>>>> the build if a number outside of the range allowed is passed.
>>>> Example:
>>>>
>>>> 	#define FOO_MASK GENMASK_U32(33, 4)
>>>>
>>>> will generate a warning like:
>>>>
>>>> 	../include/linux/bits.h:41:31: error: left shift count >= width of type [-Werror=shift-count-overflow]
>>>> 	   41 |          (((t)~0ULL - ((t)(1) << (l)) + 1) & \
>>>> 	      |                               ^~
>>>>
>>>> Signed-off-by: Yury Norov <yury.norov@xxxxxxxxx>
>>>> Signed-off-by: Lucas De Marchi <lucas.demarchi@xxxxxxxxx>
>>>> Acked-by: Jani Nikula <jani.nikula@xxxxxxxxx>
>>>> Signed-off-by: Vincent Mailhol <mailhol.vincent@xxxxxxxxxx>
>>>> ---
>>>> Changelog:
>>>>
>>>>   v3 -> v4:
>>>>
>>>>     - The v3 is one year old. Meanwhile people started using
>>>>       __GENMASK() directly. So instead of generalizing __GENMASK() to
>>>>       support different types, add a new GENMASK_t().
>>>>
>>>>     - replace ~0ULL by ~_ULL(0). Otherwise, __GENMASK_t() would fail
>>>>       in asm code.
>>>>
>>>>     - Make GENMASK_U8() and GENMASK_U16() return an unsigned int. In
>>>>       v3, due to the integer promotion rules, these were returning a
>>>>       signed integer. By casting these to unsigned int, at least the
>>>>       signedness is kept.
>>>> ---
>>>>  include/linux/bitops.h |  1 -
>>>>  include/linux/bits.h   | 33 +++++++++++++++++++++++++++++----
>>>>  2 files changed, 29 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/include/linux/bitops.h b/include/linux/bitops.h
>>>> index c1cb53cf2f0f8662ed3e324578f74330e63f935d..9be2d50da09a417966b3d11c84092bb2f4cd0bef 100644
>>>> --- a/include/linux/bitops.h
>>>> +++ b/include/linux/bitops.h
>>>> @@ -8,7 +8,6 @@
>>>>  
>>>>  #include <uapi/linux/kernel.h>
>>>>  
>>>> -#define BITS_PER_TYPE(type)	(sizeof(type) * BITS_PER_BYTE)
>>>>  #define BITS_TO_LONGS(nr)	__KERNEL_DIV_ROUND_UP(nr, BITS_PER_TYPE(long))
>>>>  #define BITS_TO_U64(nr)		__KERNEL_DIV_ROUND_UP(nr, BITS_PER_TYPE(u64))
>>>>  #define BITS_TO_U32(nr)		__KERNEL_DIV_ROUND_UP(nr, BITS_PER_TYPE(u32))
>>>> diff --git a/include/linux/bits.h b/include/linux/bits.h
>>>> index 5f68980a1b98d771426872c74d7b5c0f79e5e802..f202e46d2f4b7899c16d975120f3fa3ae41556ae 100644
>>>> --- a/include/linux/bits.h
>>>> +++ b/include/linux/bits.h
>>>> @@ -12,6 +12,7 @@
>>>>  #define BIT_ULL_MASK(nr)	(ULL(1) << ((nr) % BITS_PER_LONG_LONG))
>>>>  #define BIT_ULL_WORD(nr)	((nr) / BITS_PER_LONG_LONG)
>>>>  #define BITS_PER_BYTE		8
>>>> +#define BITS_PER_TYPE(type)	(sizeof(type) * BITS_PER_BYTE)
>>>>  
>>>>  /*
>>>>   * Create a contiguous bitmask starting at bit position @l and ending at
>>>> @@ -25,14 +26,38 @@
>>>>  
>>>>  #define GENMASK_INPUT_CHECK(h, l) BUILD_BUG_ON_ZERO(const_true((l) > (h)))
>>>>  
>>>> -#define GENMASK(h, l) \
>>>> -	(GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
>>>> -#define GENMASK_ULL(h, l) \
>>>> -	(GENMASK_INPUT_CHECK(h, l) + __GENMASK_ULL(h, l))
>>>> +/*
>>>> + * Generate a mask for the specified type @t. Additional checks are made to
>>>> + * guarantee the value returned fits in that type, relying on
>>>> + * shift-count-overflow compiler check to detect incompatible arguments.
>>>> + * For example, all these create build errors or warnings:
>>>> + *
>>>> + * - GENMASK(15, 20): wrong argument order
>>>> + * - GENMASK(72, 15): doesn't fit unsigned long
>>>> + * - GENMASK_U32(33, 15): doesn't fit in a u32
>>>> + */
>>>> +#define GENMASK_t(t, h, l)				\
>>>> +	(GENMASK_INPUT_CHECK(h, l) +			\
>>>> +	 (((t)~ULL(0) - ((t)1 << (l)) + 1) &		\
>>>> +	  ((t)~ULL(0) >> (BITS_PER_TYPE(t) - 1 - (h)))))
>>>> +
>>>> +#define GENMASK(h, l) GENMASK_t(unsigned long,  h, l)
>>>> +#define GENMASK_ULL(h, l) GENMASK_t(unsigned long long, h, l)
>>>>  
>>>>  /*
>>>>   * Missing asm support
>>>>   *
>>>> + * __GENMASK_U*() depends on BITS_PER_TYPE() which would not work in the asm
>>>> + * code as BITS_PER_TYPE() relies on sizeof(), something not available in
>>>> + * asm. Nethertheless, the concept of fixed width integers is a C thing which
>>>> + * does not apply to assembly code.
>>>> + */
>>>> +#define GENMASK_U8(h, l) ((unsigned int)GENMASK_t(u8,  h, l))
>>>> +#define GENMASK_U16(h, l) ((unsigned int)GENMASK_t(u16, h, l))
>>>> +#define GENMASK_U32(h, l) GENMASK_t(u32, h, l)
>>>> +#define GENMASK_U64(h, l) GENMASK_t(u64, h, l)
>>>> +
>>>> +/*
>>>>   * __GENMASK_U128() depends on _BIT128() which would not work
>>>>   * in the asm code, as it shifts an 'unsigned __int128' data
>>>>   * type instead of direct representation of 128 bit constants
>>>>
>>>> -- 
>>>> 2.45.3
>>>>
>