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Message-ID: <1dd33784-e9f8-46ca-bf6c-861ba9600e76@arm.com>
Date: Mon, 19 Aug 2024 08:48:41 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Yury Norov <yury.norov@...il.com>
Cc: linux-kernel@...r.kernel.org, ardb@...nel.org,
Andrew Morton <akpm@...ux-foundation.org>,
Rasmus Villemoes <linux@...musvillemoes.dk>, Arnd Bergmann <arnd@...db.de>,
linux-arch@...r.kernel.org
Subject: Re: [PATCH V3 1/2] uapi: Define GENMASK_U128
On 8/17/24 19:27, Yury Norov wrote:
> On Fri, Aug 16, 2024 at 11:58:04AM +0530, Anshuman Khandual wrote:
>>
>>
>> On 8/1/24 12:46, Anshuman Khandual wrote:
>>> This adds GENMASK_U128() and __GENMASK_U128() macros using __BITS_PER_U128
>>> and __int128 data types. These macros will be used in providing support for
>>> generating 128 bit masks.
>>>
>>> Cc: Yury Norov <yury.norov@...il.com>
>>> Cc: Rasmus Villemoes <linux@...musvillemoes.dk>
>>> Cc: Arnd Bergmann <arnd@...db.de>>
>>> Cc: linux-kernel@...r.kernel.org
>>> Cc: linux-arch@...r.kernel.org
>>> Reviewed-by: Arnd Bergmann <arnd@...db.de>
>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
>>> ---
>>> include/linux/bits.h | 13 +++++++++++++
>>> include/uapi/linux/bits.h | 3 +++
>>> include/uapi/linux/const.h | 15 +++++++++++++++
>>> 3 files changed, 31 insertions(+)
>>>
>>> diff --git a/include/linux/bits.h b/include/linux/bits.h
>>> index 0eb24d21aac2..bf99feb5570e 100644
>>> --- a/include/linux/bits.h
>>> +++ b/include/linux/bits.h
>>> @@ -36,4 +36,17 @@
>>> #define GENMASK_ULL(h, l) \
>>> (GENMASK_INPUT_CHECK(h, l) + __GENMASK_ULL(h, l))
>>>
>>> +/*
>>> + * Missing asm support
>>> + *
>>> + * __GENMASK_U128() depends on _BIT128() which would not work
>>> + * in the asm code, as it shifts an 'unsigned __init128' data
>>> + * type instead of direct representation of 128 bit constants
>>> + * such as long and unsigned long. The fundamental problem is
>>> + * that a 128 bit constant will get silently truncated by the
>>> + * gcc compiler.
>>> + */
>>> +#define GENMASK_U128(h, l) \
>>> + (GENMASK_INPUT_CHECK(h, l) + __GENMASK_U128(h, l))
>>> +
>>> #endif /* __LINUX_BITS_H */
>>> diff --git a/include/uapi/linux/bits.h b/include/uapi/linux/bits.h
>>> index 3c2a101986a3..4d4b7b08003c 100644
>>> --- a/include/uapi/linux/bits.h
>>> +++ b/include/uapi/linux/bits.h
>>> @@ -12,4 +12,7 @@
>>> (((~_ULL(0)) - (_ULL(1) << (l)) + 1) & \
>>> (~_ULL(0) >> (__BITS_PER_LONG_LONG - 1 - (h))))
>>>
>>> +#define __GENMASK_U128(h, l) \
>>> + ((_BIT128((h) + 1)) - (_BIT128(l)))
>>> +
>>> #endif /* _UAPI_LINUX_BITS_H */
>>> diff --git a/include/uapi/linux/const.h b/include/uapi/linux/const.h
>>> index a429381e7ca5..5be12e8f8f9c 100644
>>> --- a/include/uapi/linux/const.h
>>> +++ b/include/uapi/linux/const.h
>>> @@ -28,6 +28,21 @@
>>> #define _BITUL(x) (_UL(1) << (x))
>>> #define _BITULL(x) (_ULL(1) << (x))
>>>
>>> +/*
>>> + * Missing asm support
>>> + *
>>> + * __BIT128() would not work in the asm code, as it shifts an
>>> + * 'unsigned __init128' data type as direct representation of
>>> + * 128 bit constants is not supported in the gcc compiler, as
>>> + * they get silently truncated.
>>> + *
>>> + * TODO: Please revisit this implementation when gcc compiler
>>> + * starts representing 128 bit constants directly like long
>>> + * and unsigned long etc. Subsequently drop the comment for
>>> + * GENMASK_U128() which would then start supporting asm code.
>>> + */
>>> +#define _BIT128(x) ((unsigned __int128)(1) << (x))
>>> +
>>> #define __ALIGN_KERNEL(x, a) __ALIGN_KERNEL_MASK(x, (__typeof__(x))(a) - 1)
>>> #define __ALIGN_KERNEL_MASK(x, mask) (((x) + (mask)) & ~(mask))
>>>
>>
>> Hello Yuri/Arnd,
>>
>> This proposed GENMASK_U128(h, l) warns during build when the higher end
>> bit is 127 (which in itself is a valid input).
>>
>> ./include/uapi/linux/const.h:45:44: warning: left shift count >= width of type [-Wshift-count-overflow]
>> 45 | #define _BIT128(x) ((unsigned __int128)(1) << (x))
>> | ^~
>> ./include/asm-generic/bug.h:123:25: note: in definition of macro ‘WARN_ON’
>> 123 | int __ret_warn_on = !!(condition); \
>> | ^~~~~~~~~
>> ./include/uapi/linux/bits.h:16:4: note: in expansion of macro ‘_BIT128’
>> 16 | ((_BIT128((h) + 1)) - (_BIT128(l)))
>> | ^~~~~~~
>> ./include/linux/bits.h:51:31: note: in expansion of macro ‘__GENMASK_U128’
>> 51 | (GENMASK_INPUT_CHECK(h, l) + __GENMASK_U128(h, l))
>> | ^~~~~~~~~~~~~~
>>
>> This is caused by ((unsigned __int128)(1) << (128)) which is generated
>> via (h + 1) element in __GENMASK_U128().
>>
>> #define _BIT128(x) ((unsigned __int128)(1) << (x))
>> #define __GENMASK_U128(h, l) \
>> ((_BIT128((h) + 1)) - (_BIT128(l)))
>>
>> Adding some extra tests in lib/test_bits.c exposes this build problem,
>> although it does not fail these new tests.
>>
>> [ 1.719221] # Subtest: bits-test
>> [ 1.719291] # module: test_bits
>> [ 1.720522] ok 1 genmask_test
>> [ 1.721570] ok 2 genmask_ull_test
>> [ 1.722668] ok 3 genmask_u128_test
>> [ 1.723760] ok 4 genmask_input_check_test
>> [ 1.723909] # bits-test: pass:4 fail:0 skip:0 total:4
>> [ 1.724101] ok 1 bits-test
>>
>> diff --git a/lib/test_bits.c b/lib/test_bits.c
>> index d3d858b24e02..7a972edc7122 100644
>> --- a/lib/test_bits.c
>> +++ b/lib/test_bits.c
>> @@ -49,6 +49,8 @@ static void genmask_u128_test(struct kunit *test)
>> KUNIT_EXPECT_EQ(test, 0xffffffffffffffffULL, GENMASK_U128(63, 0));
>> KUNIT_EXPECT_EQ(test, 0xffffffffffffffffULL, GENMASK_U128(64, 0) >> 1);
>> KUNIT_EXPECT_EQ(test, 0x00000000ffffffffULL, GENMASK_U128(81, 50) >> 50);
>> + KUNIT_EXPECT_EQ(test, 0x0000000000000003ULL, GENMASK_U128(127, 126) >> 126);
>> + KUNIT_EXPECT_EQ(test, 0x0000000000000001ULL, GENMASK_U128(127, 127) >> 127);
>>
>> The most significant bit in the generate mask can be added separately
>> , thus voiding that extra shift. The following patch solves the build
>> problem.
>>
>> diff --git a/include/uapi/linux/bits.h b/include/uapi/linux/bits.h
>> index 4d4b7b08003c..4e50f635c6d9 100644
>> --- a/include/uapi/linux/bits.h
>> +++ b/include/uapi/linux/bits.h
>> @@ -13,6 +13,6 @@
>> (~_ULL(0) >> (__BITS_PER_LONG_LONG - 1 - (h))))
>>
>> #define __GENMASK_U128(h, l) \
>> - ((_BIT128((h) + 1)) - (_BIT128(l)))
>> + (((_BIT128(h)) - (_BIT128(l))) | (_BIT128(h)))
>
> Can you send v3 with the fix? I will drop this series from bitmap-for-next
> meanwhile.
Sure, will send out V4 (current series being V3).
>
> Can you also extend the test for more? I'd like to check for example
> the (127, 0) range. Also, can you please check both HI and LO parts
> the mask?
Following tests form the complete set on GENMASK_U128(). The last four tests
here will be added in V4. Please also note that only 64 bit mask portion can
be tested at once.
KUNIT_EXPECT_EQ(test, 0x0000000000ff0000ULL, GENMASK_U128(87, 80) >> 64);
KUNIT_EXPECT_EQ(test, 0x0000000000ffffffULL, GENMASK_U128(87, 64) >> 64);
KUNIT_EXPECT_EQ(test, 0x0000000000000001ULL, GENMASK_U128(0, 0));
KUNIT_EXPECT_EQ(test, 0xffffffffffffffffULL, GENMASK_U128(63, 0));
KUNIT_EXPECT_EQ(test, 0xffffffffffffffffULL, GENMASK_U128(64, 0) >> 1);
KUNIT_EXPECT_EQ(test, 0x00000000ffffffffULL, GENMASK_U128(81, 50) >> 50);
KUNIT_EXPECT_EQ(test, 0x0000000000000003ULL, GENMASK_U128(127, 126) >> 126);
KUNIT_EXPECT_EQ(test, 0x0000000000000001ULL, GENMASK_U128(127, 127) >> 127);
KUNIT_EXPECT_EQ(test, 0xffffffffffffffffULL, GENMASK_U128(127, 0) >> 64);
KUNIT_EXPECT_EQ(test, 0xffffffffffffffffULL, GENMASK_U128(127, 0) & ~GENMASK_U128(127, 64));
Although, please do let me know if you would like to add some more tests.
>
> For the v3, please share the link to the following series that
> actually uses new API.
Sure, will add the following link pointing to the work in progress on arm64.
https://lore.kernel.org/all/20240801054436.612024-1-anshuman.khandual@arm.com/
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