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Message-ID: <090eb237-10f4-4358-be07-1eb8d30c3ec1@arm.com>
Date: Fri, 16 Aug 2024 11:58:04 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: linux-kernel@...r.kernel.org
Cc: ardb@...nel.org, Andrew Morton <akpm@...ux-foundation.org>,
Yury Norov <yury.norov@...il.com>,
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 8/1/24 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)))
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