[tip: x86/alternatives] x86/lib/memmove: Decouple ERMS from FSRM

[tip-bot2 for Borislav Petkov (AMD)]

The following commit has been merged into the x86/alternatives branch of tip:

Commit-ID:     14e4ec9c3e9164c6719f98d8a3065c487be2aaa5
Gitweb:        https://git.kernel.org/tip/14e4ec9c3e9164c6719f98d8a3065c487be2aaa5
Author:        Borislav Petkov (AMD) <bp@xxxxxxxxx>
AuthorDate:    Sun, 26 Feb 2023 21:04:26 +01:00
Committer:     Borislav Petkov (AMD) <bp@xxxxxxxxx>
CommitterDate: Wed, 10 May 2023 14:51:56 +02:00

x86/lib/memmove: Decouple ERMS from FSRM

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@xxxxxxxxxxxx>
Reported-by: Jiri Slaby <jirislaby@xxxxxxxxxx>
Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Link: https://lore.kernel.org/r/Y/yK0dyzI0MMdTie@xxxxxxx
---
 arch/x86/lib/memmove_64.S | 13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/arch/x86/lib/memmove_64.S b/arch/x86/lib/memmove_64.S
index 0266186..0559b20 100644
--- a/arch/x86/lib/memmove_64.S
+++ b/arch/x86/lib/memmove_64.S
@@ -38,10 +38,12 @@ 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
@@ -207,11 +209,6 @@ SYM_FUNC_START(__memmove)
 	movb %r11b, (%rdi)
 13:
 	RET
-
-.Lmemmove_erms:
-	movq %rdx, %rcx
-	rep movsb
-	RET
 SYM_FUNC_END(__memmove)
 EXPORT_SYMBOL(__memmove)