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, 91 insertions, 0 deletions

diff --git a/arch/arm/lib/io-writesb.S b/arch/arm/lib/io-writesb.S
new file mode 100644
index 000000000..e2ae312f0
--- /dev/null
+++ b/arch/arm/lib/io-writesb.S
@@ -0,0 +1,91 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ *  linux/arch/arm/lib/io-writesb.S
+ *
+ *  Copyright (C) 1995-2000 Russell King
+ */
+#include <linux/linkage.h>
+#include <asm/assembler.h>
+
+		.macro	outword, rd
+#ifndef __ARMEB__
+		strb	\rd, [r0]
+		mov	\rd, \rd, lsr #8
+		strb	\rd, [r0]
+		mov	\rd, \rd, lsr #8
+		strb	\rd, [r0]
+		mov	\rd, \rd, lsr #8
+		strb	\rd, [r0]
+#else
+		mov	lr, \rd, lsr #24
+		strb	lr, [r0]
+		mov	lr, \rd, lsr #16
+		strb	lr, [r0]
+		mov	lr, \rd, lsr #8
+		strb	lr, [r0]
+		strb	\rd, [r0]
+#endif
+		.endm
+
+.Loutsb_align:	rsb	ip, ip, #4
+		cmp	ip, r2
+		movgt	ip, r2
+		cmp	ip, #2
+		ldrb	r3, [r1], #1
+		strb	r3, [r0]
+		ldrbge	r3, [r1], #1
+		strbge	r3, [r0]
+		ldrbgt	r3, [r1], #1
+		strbgt	r3, [r0]
+		subs	r2, r2, ip
+		bne	.Loutsb_aligned
+
+ENTRY(__raw_writesb)
+		teq	r2, #0		@ do we have to check for the zero len?
+		reteq	lr
+		ands	ip, r1, #3
+		bne	.Loutsb_align
+
+.Loutsb_aligned:
+		stmfd	sp!, {r4, r5, lr}
+
+		subs	r2, r2, #16
+		bmi	.Loutsb_no_16
+
+.Loutsb_16_lp:	ldmia	r1!, {r3, r4, r5, ip}
+		outword	r3
+		outword	r4
+		outword	r5
+		outword	ip
+		subs	r2, r2, #16
+		bpl	.Loutsb_16_lp
+
+		tst	r2, #15
+		ldmfdeq	sp!, {r4, r5, pc}
+
+.Loutsb_no_16:	tst	r2, #8
+		beq	.Loutsb_no_8
+
+		ldmia	r1!, {r3, r4}
+		outword	r3
+		outword	r4
+
+.Loutsb_no_8:	tst	r2, #4
+		beq	.Loutsb_no_4
+
+		ldr	r3, [r1], #4
+		outword	r3
+
+.Loutsb_no_4:	ands	r2, r2, #3
+		ldmfdeq	sp!, {r4, r5, pc}
+
+		cmp	r2, #2
+		ldrb	r3, [r1], #1
+		strb	r3, [r0]
+		ldrbge	r3, [r1], #1
+		strbge	r3, [r0]
+		ldrbgt	r3, [r1]
+		strbgt	r3, [r0]
+
+		ldmfd	sp!, {r4, r5, pc}
+ENDPROC(__raw_writesb)