kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 605 insertions, 0 deletions

diff --git a/arch/alpha/lib/ev6-memset.S b/arch/alpha/lib/ev6-memset.S
new file mode 100644
index 000000000..89d7809da
--- /dev/null
+++ b/arch/alpha/lib/ev6-memset.S
@@ -0,0 +1,605 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * arch/alpha/lib/ev6-memset.S
+ *
+ * This is an efficient (and relatively small) implementation of the C library
+ * "memset()" function for the 21264 implementation of Alpha.
+ *
+ * 21264 version  contributed by Rick Gorton <rick.gorton@alpha-processor.com>
+ *
+ * Much of the information about 21264 scheduling/coding comes from:
+ *	Compiler Writer's Guide for the Alpha 21264
+ *	abbreviated as 'CWG' in other comments here
+ *	ftp.digital.com/pub/Digital/info/semiconductor/literature/dsc-library.html
+ * Scheduling notation:
+ *	E	- either cluster
+ *	U	- upper subcluster; U0 - subcluster U0; U1 - subcluster U1
+ *	L	- lower subcluster; L0 - subcluster L0; L1 - subcluster L1
+ * The algorithm for the leading and trailing quadwords remains the same,
+ * however the loop has been unrolled to enable better memory throughput,
+ * and the code has been replicated for each of the entry points: __memset
+ * and __memset16 to permit better scheduling to eliminate the stalling
+ * encountered during the mask replication.
+ * A future enhancement might be to put in a byte store loop for really
+ * small (say < 32 bytes) memset()s.  Whether or not that change would be
+ * a win in the kernel would depend upon the contextual usage.
+ * WARNING: Maintaining this is going to be more work than the above version,
+ * as fixes will need to be made in multiple places.  The performance gain
+ * is worth it.
+ */
+#include <linux/export.h>
+	.set noat
+	.set noreorder
+.text
+	.globl memset
+	.globl __memset
+	.globl ___memset
+	.globl __memset16
+	.globl __constant_c_memset
+
+	.ent ___memset
+.align 5
+___memset:
+	.frame $30,0,$26,0
+	.prologue 0
+
+	/*
+	 * Serious stalling happens.  The only way to mitigate this is to
+	 * undertake a major re-write to interleave the constant materialization
+	 * with other parts of the fall-through code.  This is important, even
+	 * though it makes maintenance tougher.
+	 * Do this later.
+	 */
+	and $17,255,$1		# E : 00000000000000ch
+	insbl $17,1,$2		# U : 000000000000ch00
+	bis $16,$16,$0		# E : return value
+	ble $18,end_b		# U : zero length requested?
+
+	addq $18,$16,$6		# E : max address to write to
+	bis	$1,$2,$17	# E : 000000000000chch
+	insbl	$1,2,$3		# U : 0000000000ch0000
+	insbl	$1,3,$4		# U : 00000000ch000000
+
+	or	$3,$4,$3	# E : 00000000chch0000
+	inswl	$17,4,$5	# U : 0000chch00000000
+	xor	$16,$6,$1	# E : will complete write be within one quadword?
+	inswl	$17,6,$2	# U : chch000000000000
+
+	or	$17,$3,$17	# E : 00000000chchchch
+	or	$2,$5,$2	# E : chchchch00000000
+	bic	$1,7,$1		# E : fit within a single quadword?
+	and	$16,7,$3	# E : Target addr misalignment
+
+	or	$17,$2,$17	# E : chchchchchchchch
+	beq	$1,within_quad_b # U :
+	nop			# E :
+	beq	$3,aligned_b	# U : target is 0mod8
+
+	/*
+	 * Target address is misaligned, and won't fit within a quadword
+	 */
+	ldq_u $4,0($16)		# L : Fetch first partial
+	bis $16,$16,$5		# E : Save the address
+	insql $17,$16,$2	# U : Insert new bytes
+	subq $3,8,$3		# E : Invert (for addressing uses)
+
+	addq $18,$3,$18		# E : $18 is new count ($3 is negative)
+	mskql $4,$16,$4		# U : clear relevant parts of the quad
+	subq $16,$3,$16		# E : $16 is new aligned destination
+	bis $2,$4,$1		# E : Final bytes
+
+	nop
+	stq_u $1,0($5)		# L : Store result
+	nop
+	nop
+
+.align 4
+aligned_b:
+	/*
+	 * We are now guaranteed to be quad aligned, with at least
+	 * one partial quad to write.
+	 */
+
+	sra $18,3,$3		# U : Number of remaining quads to write
+	and $18,7,$18		# E : Number of trailing bytes to write
+	bis $16,$16,$5		# E : Save dest address
+	beq $3,no_quad_b	# U : tail stuff only
+
+	/*
+	 * it's worth the effort to unroll this and use wh64 if possible
+	 * Lifted a bunch of code from clear_user.S
+	 * At this point, entry values are:
+	 * $16	Current destination address
+	 * $5	A copy of $16
+	 * $6	The max quadword address to write to
+	 * $18	Number trailer bytes
+	 * $3	Number quads to write
+	 */
+
+	and	$16, 0x3f, $2	# E : Forward work (only useful for unrolled loop)
+	subq	$3, 16, $4	# E : Only try to unroll if > 128 bytes
+	subq	$2, 0x40, $1	# E : bias counter (aligning stuff 0mod64)
+	blt	$4, loop_b	# U :
+
+	/*
+	 * We know we've got at least 16 quads, minimum of one trip
+	 * through unrolled loop.  Do a quad at a time to get us 0mod64
+	 * aligned.
+	 */
+
+	nop			# E :
+	nop			# E :
+	nop			# E :
+	beq	$1, $bigalign_b	# U :
+
+$alignmod64_b:
+	stq	$17, 0($5)	# L :
+	subq	$3, 1, $3	# E : For consistency later
+	addq	$1, 8, $1	# E : Increment towards zero for alignment
+	addq	$5, 8, $4	# E : Initial wh64 address (filler instruction)
+
+	nop
+	nop
+	addq	$5, 8, $5	# E : Inc address
+	blt	$1, $alignmod64_b # U :
+
+$bigalign_b:
+	/*
+	 * $3 - number quads left to go
+	 * $5 - target address (aligned 0mod64)
+	 * $17 - mask of stuff to store
+	 * Scratch registers available: $7, $2, $4, $1
+	 * we know that we'll be taking a minimum of one trip through
+ 	 * CWG Section 3.7.6: do not expect a sustained store rate of > 1/cycle
+	 * Assumes the wh64 needs to be for 2 trips through the loop in the future
+	 * The wh64 is issued on for the starting destination address for trip +2
+	 * through the loop, and if there are less than two trips left, the target
+	 * address will be for the current trip.
+	 */
+
+$do_wh64_b:
+	wh64	($4)		# L1 : memory subsystem write hint
+	subq	$3, 24, $2	# E : For determining future wh64 addresses
+	stq	$17, 0($5)	# L :
+	nop			# E :
+
+	addq	$5, 128, $4	# E : speculative target of next wh64
+	stq	$17, 8($5)	# L :
+	stq	$17, 16($5)	# L :
+	addq	$5, 64, $7	# E : Fallback address for wh64 (== next trip addr)
+
+	stq	$17, 24($5)	# L :
+	stq	$17, 32($5)	# L :
+	cmovlt	$2, $7, $4	# E : Latency 2, extra mapping cycle
+	nop
+
+	stq	$17, 40($5)	# L :
+	stq	$17, 48($5)	# L :
+	subq	$3, 16, $2	# E : Repeat the loop at least once more?
+	nop
+
+	stq	$17, 56($5)	# L :
+	addq	$5, 64, $5	# E :
+	subq	$3, 8, $3	# E :
+	bge	$2, $do_wh64_b	# U :
+
+	nop
+	nop
+	nop
+	beq	$3, no_quad_b	# U : Might have finished already
+
+.align 4
+	/*
+	 * Simple loop for trailing quadwords, or for small amounts
+	 * of data (where we can't use an unrolled loop and wh64)
+	 */
+loop_b:
+	stq $17,0($5)		# L :
+	subq $3,1,$3		# E : Decrement number quads left
+	addq $5,8,$5		# E : Inc address
+	bne $3,loop_b		# U : more?
+
+no_quad_b:
+	/*
+	 * Write 0..7 trailing bytes.
+	 */
+	nop			# E :
+	beq $18,end_b		# U : All done?
+	ldq $7,0($5)		# L :
+	mskqh $7,$6,$2		# U : Mask final quad
+
+	insqh $17,$6,$4		# U : New bits
+	bis $2,$4,$1		# E : Put it all together
+	stq $1,0($5)		# L : And back to memory
+	ret $31,($26),1		# L0 :
+
+within_quad_b:
+	ldq_u $1,0($16)		# L :
+	insql $17,$16,$2	# U : New bits
+	mskql $1,$16,$4		# U : Clear old
+	bis $2,$4,$2		# E : New result
+
+	mskql $2,$6,$4		# U :
+	mskqh $1,$6,$2		# U :
+	bis $2,$4,$1		# E :
+	stq_u $1,0($16)		# L :
+
+end_b:
+	nop
+	nop
+	nop
+	ret $31,($26),1		# L0 :
+	.end ___memset
+	EXPORT_SYMBOL(___memset)
+
+	/*
+	 * This is the original body of code, prior to replication and
+	 * rescheduling.  Leave it here, as there may be calls to this
+	 * entry point.
+	 */
+.align 4
+	.ent __constant_c_memset
+__constant_c_memset:
+	.frame $30,0,$26,0
+	.prologue 0
+
+	addq $18,$16,$6		# E : max address to write to
+	bis $16,$16,$0		# E : return value
+	xor $16,$6,$1		# E : will complete write be within one quadword?
+	ble $18,end		# U : zero length requested?
+
+	bic $1,7,$1		# E : fit within a single quadword
+	beq $1,within_one_quad	# U :
+	and $16,7,$3		# E : Target addr misalignment
+	beq $3,aligned		# U : target is 0mod8
+
+	/*
+	 * Target address is misaligned, and won't fit within a quadword
+	 */
+	ldq_u $4,0($16)		# L : Fetch first partial
+	bis $16,$16,$5		# E : Save the address
+	insql $17,$16,$2	# U : Insert new bytes
+	subq $3,8,$3		# E : Invert (for addressing uses)
+
+	addq $18,$3,$18		# E : $18 is new count ($3 is negative)
+	mskql $4,$16,$4		# U : clear relevant parts of the quad
+	subq $16,$3,$16		# E : $16 is new aligned destination
+	bis $2,$4,$1		# E : Final bytes
+
+	nop
+	stq_u $1,0($5)		# L : Store result
+	nop
+	nop
+
+.align 4
+aligned:
+	/*
+	 * We are now guaranteed to be quad aligned, with at least
+	 * one partial quad to write.
+	 */
+
+	sra $18,3,$3		# U : Number of remaining quads to write
+	and $18,7,$18		# E : Number of trailing bytes to write
+	bis $16,$16,$5		# E : Save dest address
+	beq $3,no_quad		# U : tail stuff only
+
+	/*
+	 * it's worth the effort to unroll this and use wh64 if possible
+	 * Lifted a bunch of code from clear_user.S
+	 * At this point, entry values are:
+	 * $16	Current destination address
+	 * $5	A copy of $16
+	 * $6	The max quadword address to write to
+	 * $18	Number trailer bytes
+	 * $3	Number quads to write
+	 */
+
+	and	$16, 0x3f, $2	# E : Forward work (only useful for unrolled loop)
+	subq	$3, 16, $4	# E : Only try to unroll if > 128 bytes
+	subq	$2, 0x40, $1	# E : bias counter (aligning stuff 0mod64)
+	blt	$4, loop	# U :
+
+	/*
+	 * We know we've got at least 16 quads, minimum of one trip
+	 * through unrolled loop.  Do a quad at a time to get us 0mod64
+	 * aligned.
+	 */
+
+	nop			# E :
+	nop			# E :
+	nop			# E :
+	beq	$1, $bigalign	# U :
+
+$alignmod64:
+	stq	$17, 0($5)	# L :
+	subq	$3, 1, $3	# E : For consistency later
+	addq	$1, 8, $1	# E : Increment towards zero for alignment
+	addq	$5, 8, $4	# E : Initial wh64 address (filler instruction)
+
+	nop
+	nop
+	addq	$5, 8, $5	# E : Inc address
+	blt	$1, $alignmod64	# U :
+
+$bigalign:
+	/*
+	 * $3 - number quads left to go
+	 * $5 - target address (aligned 0mod64)
+	 * $17 - mask of stuff to store
+	 * Scratch registers available: $7, $2, $4, $1
+	 * we know that we'll be taking a minimum of one trip through
+ 	 * CWG Section 3.7.6: do not expect a sustained store rate of > 1/cycle
+	 * Assumes the wh64 needs to be for 2 trips through the loop in the future
+	 * The wh64 is issued on for the starting destination address for trip +2
+	 * through the loop, and if there are less than two trips left, the target
+	 * address will be for the current trip.
+	 */
+
+$do_wh64:
+	wh64	($4)		# L1 : memory subsystem write hint
+	subq	$3, 24, $2	# E : For determining future wh64 addresses
+	stq	$17, 0($5)	# L :
+	nop			# E :
+
+	addq	$5, 128, $4	# E : speculative target of next wh64
+	stq	$17, 8($5)	# L :
+	stq	$17, 16($5)	# L :
+	addq	$5, 64, $7	# E : Fallback address for wh64 (== next trip addr)
+
+	stq	$17, 24($5)	# L :
+	stq	$17, 32($5)	# L :
+	cmovlt	$2, $7, $4	# E : Latency 2, extra mapping cycle
+	nop
+
+	stq	$17, 40($5)	# L :
+	stq	$17, 48($5)	# L :
+	subq	$3, 16, $2	# E : Repeat the loop at least once more?
+	nop
+
+	stq	$17, 56($5)	# L :
+	addq	$5, 64, $5	# E :
+	subq	$3, 8, $3	# E :
+	bge	$2, $do_wh64	# U :
+
+	nop
+	nop
+	nop
+	beq	$3, no_quad	# U : Might have finished already
+
+.align 4
+	/*
+	 * Simple loop for trailing quadwords, or for small amounts
+	 * of data (where we can't use an unrolled loop and wh64)
+	 */
+loop:
+	stq $17,0($5)		# L :
+	subq $3,1,$3		# E : Decrement number quads left
+	addq $5,8,$5		# E : Inc address
+	bne $3,loop		# U : more?
+
+no_quad:
+	/*
+	 * Write 0..7 trailing bytes.
+	 */
+	nop			# E :
+	beq $18,end		# U : All done?
+	ldq $7,0($5)		# L :
+	mskqh $7,$6,$2		# U : Mask final quad
+
+	insqh $17,$6,$4		# U : New bits
+	bis $2,$4,$1		# E : Put it all together
+	stq $1,0($5)		# L : And back to memory
+	ret $31,($26),1		# L0 :
+
+within_one_quad:
+	ldq_u $1,0($16)		# L :
+	insql $17,$16,$2	# U : New bits
+	mskql $1,$16,$4		# U : Clear old
+	bis $2,$4,$2		# E : New result
+
+	mskql $2,$6,$4		# U :
+	mskqh $1,$6,$2		# U :
+	bis $2,$4,$1		# E :
+	stq_u $1,0($16)		# L :
+
+end:
+	nop
+	nop
+	nop
+	ret $31,($26),1		# L0 :
+	.end __constant_c_memset
+	EXPORT_SYMBOL(__constant_c_memset)
+
+	/*
+	 * This is a replicant of the __constant_c_memset code, rescheduled
+	 * to mask stalls.  Note that entry point names also had to change
+	 */
+	.align 5
+	.ent __memset16
+
+__memset16:
+	.frame $30,0,$26,0
+	.prologue 0
+
+	inswl $17,0,$5		# U : 000000000000c1c2
+	inswl $17,2,$2		# U : 00000000c1c20000
+	bis $16,$16,$0		# E : return value
+	addq	$18,$16,$6	# E : max address to write to
+
+	ble $18, end_w		# U : zero length requested?
+	inswl	$17,4,$3	# U : 0000c1c200000000
+	inswl	$17,6,$4	# U : c1c2000000000000
+	xor	$16,$6,$1	# E : will complete write be within one quadword?
+
+	or	$2,$5,$2	# E : 00000000c1c2c1c2
+	or	$3,$4,$17	# E : c1c2c1c200000000
+	bic	$1,7,$1		# E : fit within a single quadword
+	and	$16,7,$3	# E : Target addr misalignment
+
+	or	$17,$2,$17	# E : c1c2c1c2c1c2c1c2
+	beq $1,within_quad_w	# U :
+	nop
+	beq $3,aligned_w	# U : target is 0mod8
+
+	/*
+	 * Target address is misaligned, and won't fit within a quadword
+	 */
+	ldq_u $4,0($16)		# L : Fetch first partial
+	bis $16,$16,$5		# E : Save the address
+	insql $17,$16,$2	# U : Insert new bytes
+	subq $3,8,$3		# E : Invert (for addressing uses)
+
+	addq $18,$3,$18		# E : $18 is new count ($3 is negative)
+	mskql $4,$16,$4		# U : clear relevant parts of the quad
+	subq $16,$3,$16		# E : $16 is new aligned destination
+	bis $2,$4,$1		# E : Final bytes
+
+	nop
+	stq_u $1,0($5)		# L : Store result
+	nop
+	nop
+
+.align 4
+aligned_w:
+	/*
+	 * We are now guaranteed to be quad aligned, with at least
+	 * one partial quad to write.
+	 */
+
+	sra $18,3,$3		# U : Number of remaining quads to write
+	and $18,7,$18		# E : Number of trailing bytes to write
+	bis $16,$16,$5		# E : Save dest address
+	beq $3,no_quad_w	# U : tail stuff only
+
+	/*
+	 * it's worth the effort to unroll this and use wh64 if possible
+	 * Lifted a bunch of code from clear_user.S
+	 * At this point, entry values are:
+	 * $16	Current destination address
+	 * $5	A copy of $16
+	 * $6	The max quadword address to write to
+	 * $18	Number trailer bytes
+	 * $3	Number quads to write
+	 */
+
+	and	$16, 0x3f, $2	# E : Forward work (only useful for unrolled loop)
+	subq	$3, 16, $4	# E : Only try to unroll if > 128 bytes
+	subq	$2, 0x40, $1	# E : bias counter (aligning stuff 0mod64)
+	blt	$4, loop_w	# U :
+
+	/*
+	 * We know we've got at least 16 quads, minimum of one trip
+	 * through unrolled loop.  Do a quad at a time to get us 0mod64
+	 * aligned.
+	 */
+
+	nop			# E :
+	nop			# E :
+	nop			# E :
+	beq	$1, $bigalign_w	# U :
+
+$alignmod64_w:
+	stq	$17, 0($5)	# L :
+	subq	$3, 1, $3	# E : For consistency later
+	addq	$1, 8, $1	# E : Increment towards zero for alignment
+	addq	$5, 8, $4	# E : Initial wh64 address (filler instruction)
+
+	nop
+	nop
+	addq	$5, 8, $5	# E : Inc address
+	blt	$1, $alignmod64_w	# U :
+
+$bigalign_w:
+	/*
+	 * $3 - number quads left to go
+	 * $5 - target address (aligned 0mod64)
+	 * $17 - mask of stuff to store
+	 * Scratch registers available: $7, $2, $4, $1
+	 * we know that we'll be taking a minimum of one trip through
+ 	 * CWG Section 3.7.6: do not expect a sustained store rate of > 1/cycle
+	 * Assumes the wh64 needs to be for 2 trips through the loop in the future
+	 * The wh64 is issued on for the starting destination address for trip +2
+	 * through the loop, and if there are less than two trips left, the target
+	 * address will be for the current trip.
+	 */
+
+$do_wh64_w:
+	wh64	($4)		# L1 : memory subsystem write hint
+	subq	$3, 24, $2	# E : For determining future wh64 addresses
+	stq	$17, 0($5)	# L :
+	nop			# E :
+
+	addq	$5, 128, $4	# E : speculative target of next wh64
+	stq	$17, 8($5)	# L :
+	stq	$17, 16($5)	# L :
+	addq	$5, 64, $7	# E : Fallback address for wh64 (== next trip addr)
+
+	stq	$17, 24($5)	# L :
+	stq	$17, 32($5)	# L :
+	cmovlt	$2, $7, $4	# E : Latency 2, extra mapping cycle
+	nop
+
+	stq	$17, 40($5)	# L :
+	stq	$17, 48($5)	# L :
+	subq	$3, 16, $2	# E : Repeat the loop at least once more?
+	nop
+
+	stq	$17, 56($5)	# L :
+	addq	$5, 64, $5	# E :
+	subq	$3, 8, $3	# E :
+	bge	$2, $do_wh64_w	# U :
+
+	nop
+	nop
+	nop
+	beq	$3, no_quad_w	# U : Might have finished already
+
+.align 4
+	/*
+	 * Simple loop for trailing quadwords, or for small amounts
+	 * of data (where we can't use an unrolled loop and wh64)
+	 */
+loop_w:
+	stq $17,0($5)		# L :
+	subq $3,1,$3		# E : Decrement number quads left
+	addq $5,8,$5		# E : Inc address
+	bne $3,loop_w		# U : more?
+
+no_quad_w:
+	/*
+	 * Write 0..7 trailing bytes.
+	 */
+	nop			# E :
+	beq $18,end_w		# U : All done?
+	ldq $7,0($5)		# L :
+	mskqh $7,$6,$2		# U : Mask final quad
+
+	insqh $17,$6,$4		# U : New bits
+	bis $2,$4,$1		# E : Put it all together
+	stq $1,0($5)		# L : And back to memory
+	ret $31,($26),1		# L0 :
+
+within_quad_w:
+	ldq_u $1,0($16)		# L :
+	insql $17,$16,$2	# U : New bits
+	mskql $1,$16,$4		# U : Clear old
+	bis $2,$4,$2		# E : New result
+
+	mskql $2,$6,$4		# U :
+	mskqh $1,$6,$2		# U :
+	bis $2,$4,$1		# E :
+	stq_u $1,0($16)		# L :
+
+end_w:
+	nop
+	nop
+	nop
+	ret $31,($26),1		# L0 :
+
+	.end __memset16
+	EXPORT_SYMBOL(__memset16)
+
+memset = ___memset
+__memset = ___memset
+	EXPORT_SYMBOL(memset)
+	EXPORT_SYMBOL(__memset)