Commit 14e4ec9c authored by Borislav Petkov (AMD)'s avatar Borislav Petkov (AMD)
Browse files

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: default avatarDaniel Verkamp <dverkamp@chromium.org>
Reported-by: default avatarJiri Slaby <jirislaby@kernel.org>
Signed-off-by: default avatarBorislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/Y/yK0dyzI0MMdTie@zn.tnic
parent 270a69c4

arch/x86/lib/memmove_64.S

+5 −8
Original line number Diff line number Diff line
@@ -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)