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, 96 insertions, 0 deletions

diff --git a/arch/alpha/lib/ev67-strncat.S b/arch/alpha/lib/ev67-strncat.S
new file mode 100644
index 000000000..8f313233e
--- /dev/null
+++ b/arch/alpha/lib/ev67-strncat.S
@@ -0,0 +1,96 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * arch/alpha/lib/ev67-strncat.S
+ * 21264 version contributed by Rick Gorton <rick.gorton@api-networks.com>
+ *
+ * Append no more than COUNT characters from the null-terminated string SRC
+ * to the null-terminated string DST.  Always null-terminate the new DST.
+ *
+ * This differs slightly from the semantics in libc in that we never write
+ * past count, whereas libc may write to count+1.  This follows the generic
+ * implementation in lib/string.c and is, IMHO, more sensible.
+ *
+ * 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
+ * Try not to change the actual algorithm if possible for consistency.
+ */
+
+#include <linux/export.h>
+	.text
+
+	.align 4
+	.globl strncat
+	.ent strncat
+strncat:
+	.frame $30, 0, $26
+	.prologue 0
+
+	mov	$16, $0		# set up return value
+	beq	$18, $zerocount	# U :
+	/* Find the end of the string.  */
+	ldq_u   $1, 0($16)	# L : load first quadword ($16 may be misaligned)
+	lda     $2, -1($31)	# E :
+
+	insqh   $2, $0, $2	# U :
+	andnot  $16, 7, $16	# E :
+	nop			# E :
+	or      $2, $1, $1	# E :
+
+	nop			# E :
+	nop			# E :
+	cmpbge  $31, $1, $2	# E : bits set iff byte == 0
+	bne     $2, $found	# U :
+
+$loop:	ldq     $1, 8($16)	# L :
+	addq    $16, 8, $16	# E :
+	cmpbge  $31, $1, $2	# E :
+	beq     $2, $loop	# U :
+
+$found:	cttz	$2, $3		# U0 :
+	addq	$16, $3, $16	# E :
+	nop			# E :
+	bsr	$23, __stxncpy	# L0 :/* Now do the append.  */
+
+	/* Worry about the null termination.  */
+
+	zapnot	$1, $27, $2	# U : was last byte a null?
+	cmplt	$27, $24, $5	# E : did we fill the buffer completely?
+	bne	$2, 0f		# U :
+	ret			# L0 :
+
+0:	or	$5, $18, $2	# E :
+	nop
+	bne	$2, 2f		# U :
+	and	$24, 0x80, $3	# E : no zero next byte
+
+	nop			# E :
+	bne	$3, 1f		# U :
+	/* Here there are bytes left in the current word.  Clear one.  */
+	addq	$24, $24, $24	# E : end-of-count bit <<= 1
+	nop			# E :
+
+2:	zap	$1, $24, $1	# U :
+	nop			# E :
+	stq_u	$1, 0($16)	# L :
+	ret			# L0 :
+
+1:	/* Here we must clear the first byte of the next DST word */
+	stb	$31, 8($16)	# L :
+	nop			# E :
+	nop			# E :
+	ret			# L0 :
+
+$zerocount:
+	nop			# E :
+	nop			# E :
+	nop			# E :
+	ret			# L0 :
+
+	.end strncat
+	EXPORT_SYMBOL(strncat)