From: "Borislav Petkov (AMD)" <bp@alien8.de>
Date: Sun, 26 Feb 2023 21:04:26 +0100
Subject: x86/lib/memmove: Decouple ERMS from FSRM
Git-commit: 14e4ec9c3e9164c6719f98d8a3065c487be2aaa5
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git#master
Patch-mainline: Queued in subsystem maintainer repository
References: bsc#1206578

Up until now it was perceived that FSRM is an improvement to ERMS and
thus it was made dependent on latter.

However, there are AMD BIOSes out there which allow for disabling of
either features and thus preventing kernels from booting due to the CMP
disappearing and thus breaking the logic in the memmove() function.

Similar observation happens on some VM migration scenarios.

Patch the proper sequences depending on which feature is enabled.

Reported-by: Daniel Verkamp <dverkamp@chromium.org>
Reported-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/Y/yK0dyzI0MMdTie@zn.tnic
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
 arch/x86/lib/memmove_64.S |   13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

--- a/arch/x86/lib/memmove_64.S
+++ b/arch/x86/lib/memmove_64.S
@@ -37,10 +37,12 @@ SYM_FUNC_START(__memmove)
 	cmp %rdi, %r8
 	jg 2f
 
-	/* FSRM implies ERMS => no length checks, do the copy directly */
+#define CHECK_LEN	cmp $0x20, %rdx; jb 1f
+#define MEMMOVE_BYTES	movq %rdx, %rcx; rep movsb; RET
 .Lmemmove_begin_forward:
-	ALTERNATIVE "cmp $0x20, %rdx; jb 1f", "", X86_FEATURE_FSRM
-	ALTERNATIVE "", "jmp .Lmemmove_erms", X86_FEATURE_ERMS
+	ALTERNATIVE_2 __stringify(CHECK_LEN), \
+		      __stringify(CHECK_LEN; MEMMOVE_BYTES), X86_FEATURE_ERMS, \
+		      __stringify(MEMMOVE_BYTES), X86_FEATURE_FSRM
 
 	/*
 	 * movsq instruction have many startup latency
@@ -206,11 +208,6 @@ SYM_FUNC_START(__memmove)
 	movb %r11b, (%rdi)
 13:
 	RET
-
-.Lmemmove_erms:
-	movq %rdx, %rcx
-	rep movsb
-	RET
 SYM_FUNC_END(__memmove)
 SYM_FUNC_END_ALIAS(memmove)
 EXPORT_SYMBOL(__memmove)