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Futex hash computation requires a mask operation with read-only after
init data that will be converted to a runtime constant in the subsequent
commit.
Introduce runtime_const_mask_32 to further optimize the mask operation
in the futex hash computation hot path. Since all the current use-cases
are of the form GENMASK(n, 0), with n > 0, a single:
ubfx w0, w0, #0, #widthm1 // w0 = w0 [widthm1:0]
instruction is used for amd64 to improve instruction dinsity and
performance.
"Arm A-profile A64 Instruction Set Architecture" manual, Sec.
"A64 -- Base Instructions" [1] for UBFX instruction highlights the
immediate "width" is encoded as width minus 1 in imms (Bits [15:10])
which is patched by __runtime_fixup_mask() once the mask is known.
If a future use case arises that needs to tackle arbitrary mask,
consider using:
movz w1, #lo16, lsl #0
movk w1, #hi16, lsl #16
to patch the 32-bit mask in the asm block and return "__ret & (val)"
from runtime_const_mask_32() which allows compiler to further optimize
the logical and operation. __runtime_fixup_ptr() already patches a
"movz, + movk lsl #16" sequence which can be reused when the need
arises.
A possible implementation for this alternate scheme can be found at [2].
Suggested-by: Samuel Holland <samuel.holland@sifive.com>
Suggested-by: Charlie Jenkins <thecharlesjenkins@gmail.com>
Assisted-by: Claude:claude-sonnet-4-6
Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Charlie Jenkins <thecharlesjenkins@gmail.com>
Tested-by: Charlie Jenkins <thecharlesjenkins@gmail.com>
Link: https://developer.arm.com/documentation/ddi0602/2026-03/Base-Instructions/ [1]
Link: https://lore.kernel.org/lkml/20260430094730.31624-4-kprateek.nayak@amd.com/ [2]
Link: https://patch.msgid.link/20260728052540.4728-4-kprateek.nayak@amd.com
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The current scheme to directly patch the kernel text for runtime
constants runs into the following issue with futex adapted to using
runtime constants on arm64:
Unable to handle kernel write to read-only memory at virtual address ...
The pc points to the *p assignment in the following call chain:
futex_init()
runtime_const_init(shift, __futex_shift)
__runtime_fixup_shift()
*p = cpu_to_le32(insn);
which suggests that core_initcall() is too late to patch the kernel text
directly unlike the "d_hash_shift" which is initialized during
vfs_caches_init_early() before the protections are in place.
Use aarch64_insn_patch_text_nosync() to patch the runtime constants
instead of doing it directly to allow runtime_const_init() slightly
later into the boot.
Since aarch64_insn_patch_text_nosync() calls caches_clean_inval_pou()
internally, __runtime_fixup_caches() ends up being redundant.
runtime_const_init() are rare and the overheads of multiple calls to
caches_clean_inval_pou() instead of batching them together should be
negligible in practice.
The cpu_to_le32() conversion of instruction isn't necessary since it is
handled later in the aarch64_insn_patch_text_nosync() call-chain:
aarch64_insn_patch_text_nosync(addr, insn)
aarch64_insn_write(addr, insn)
__aarch64_insn_write(addr, cpu_to_le32(insn))
Sashiko noted that aarch64_insn_patch_text_nosync() does not expect a
lm_alias() address and Catalin suggested it is safe to drop the
lm_alias() for runtime patching since the kernel text is readable. The
address passed to fixup function is interpreted as a __le32 and
dereferenced as is to read the opcode at the patch site.
No functional changes are intended.
Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Charlie Jenkins <thecharlesjenkins@gmail.com>
Tested-by: Charlie Jenkins <thecharlesjenkins@gmail.com>
Link: https://patch.msgid.link/20260728052540.4728-3-kprateek.nayak@amd.com
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Similar as commit 284922f4c563 ("x86: uaccess: don't use runtime-const
rewriting in modules") does, make arm64's runtime const not usable by
modules too, to "make sure this doesn't get forgotten the next time
somebody wants to do runtime constant optimizations". The reason is
well explained in the above commit: "The runtime-const infrastructure
was never designed to handle the modular case, because the constant
fixup is only done at boot time for core kernel code."
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>
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This implements the runtime constant infrastructure for arm64, allowing
the dcache d_hash() function to be generated using as a constant for
hash table address followed by shift by a constant of the hash index.
[ Fixed up to deal with the big-endian case as per Mark Rutland ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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