KVM: Don't pre-reserve xarray entries when storing empty/NULL attributesgrafted

Skip the xarray reservation loop when clearing all memory attributes, as
storing NULL only erases the entry and never needs to allocate, so no
reservation (and no cleanup of a failed one) is required in that case.

Suggested-by: Sean Christopherson <seanjc@google.com>
Cc: David Ballesteros <davimaba.v@proton.me>
Signed-off-by: Zeng Chi <zengchi@kylinos.cn>
Link: https://patch.msgid.link/20260921102442.1232375-1-zeng_chi911@163.com
[sean: split to separate patch]
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 89 insertions, 0 deletions

diff --git a/arch/alpha/lib/strrchr.S b/arch/alpha/lib/strrchr.S
new file mode 100644
index 000000000..dd8e073b6
--- /dev/null
+++ b/arch/alpha/lib/strrchr.S
@@ -0,0 +1,89 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * arch/alpha/lib/strrchr.S
+ * Contributed by Richard Henderson (rth@tamu.edu)
+ *
+ * Return the address of the last occurrence of a given character
+ * within a null-terminated string, or null if it is not found.
+ */
+#include <linux/export.h>
+#include <asm/regdef.h>
+
+	.set noreorder
+	.set noat
+
+	.align 3
+	.ent strrchr
+	.globl strrchr
+strrchr:
+	.frame sp, 0, ra
+	.prologue 0
+
+	zapnot	a1, 1, a1	# e0    : zero extend our test character
+	mov	zero, t6	# .. e1 : t6 is last match aligned addr
+	sll	a1, 8, t5	# e0    : replicate our test character
+	mov	zero, t8	# .. e1 : t8 is last match byte compare mask
+	or	t5, a1, a1	# e0    :
+	ldq_u   t0, 0(a0)	# .. e1 : load first quadword
+	sll	a1, 16, t5	# e0    :
+	andnot  a0, 7, v0	# .. e1 : align source addr
+	or	t5, a1, a1	# e0    :
+	lda	t4, -1		# .. e1 : build garbage mask
+	sll	a1, 32, t5	# e0    :
+	cmpbge  zero, t0, t1	# .. e1 : bits set iff byte == zero
+	mskqh	t4, a0, t4	# e0    :
+	or	t5, a1, a1	# .. e1 : character replication complete
+	xor	t0, a1, t2	# e0    : make bytes == c zero
+	cmpbge	zero, t4, t4	# .. e1 : bits set iff byte is garbage
+	cmpbge  zero, t2, t3	# e0    : bits set iff byte == c
+	andnot	t1, t4, t1	# .. e1 : clear garbage from null test
+	andnot	t3, t4, t3	# e0    : clear garbage from char test
+	bne	t1, $eos	# .. e1 : did we already hit the terminator?
+
+	/* Character search main loop */
+$loop:
+	ldq	t0, 8(v0)	# e0    : load next quadword
+	cmovne	t3, v0, t6	# .. e1 : save previous comparisons match
+	cmovne	t3, t3, t8	# e0    :
+	addq	v0, 8, v0	# .. e1 :
+	xor	t0, a1, t2	# e0    :
+	cmpbge	zero, t0, t1	# .. e1 : bits set iff byte == zero
+	cmpbge	zero, t2, t3	# e0    : bits set iff byte == c
+	beq	t1, $loop	# .. e1 : if we havnt seen a null, loop
+
+	/* Mask out character matches after terminator */
+$eos:
+	negq	t1, t4		# e0    : isolate first null byte match
+	and	t1, t4, t4	# e1    :
+	subq	t4, 1, t5	# e0    : build a mask of the bytes up to...
+	or	t4, t5, t4	# e1    : ... and including the null
+
+	and	t3, t4, t3	# e0    : mask out char matches after null
+	cmovne	t3, t3, t8	# .. e1 : save it, if match found
+	cmovne	t3, v0, t6	# e0    :
+
+	/* Locate the address of the last matched character */
+
+	/* Retain the early exit for the ev4 -- the ev5 mispredict penalty
+	   is 5 cycles -- the same as just falling through.  */
+	beq	t8, $retnull	# .. e1 :
+
+	and	t8, 0xf0, t2	# e0    : binary search for the high bit set
+	cmovne	t2, t2, t8	# .. e1 (zdb)
+	cmovne	t2, 4, t2	# e0    :
+	and	t8, 0xcc, t1	# .. e1 :
+	cmovne	t1, t1, t8	# e0    :
+	cmovne	t1, 2, t1	# .. e1 :
+	and	t8, 0xaa, t0	# e0    :
+	cmovne	t0, 1, t0	# .. e1 (zdb)
+	addq	t2, t1, t1	# e0    :
+	addq	t6, t0, v0	# .. e1 : add our aligned base ptr to the mix
+	addq	v0, t1, v0	# e0    :
+	ret			# .. e1 :
+
+$retnull:
+	mov	zero, v0	# e0    :
+	ret			# .. e1 :
+
+	.end strrchr
+	EXPORT_SYMBOL(strrchr)