linux-kernel - Re: [PATCH 10/14] x86/alternative: Convert ALTERNATIVE

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Date: Sat, 1 Jun 2024 11:04:35 +0200
From: Borislav Petkov <bp@...en8.de>
To: Brian Gerst <brgerst@...il.com>
Cc: Borislav Petkov <bp@...nel.org>, X86 ML <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 10/14] x86/alternative: Convert ALTERNATIVE_3()

On Fri, May 31, 2024 at 01:00:55PM -0400, Brian Gerst wrote:
> Just add a label at the start of this macro, so it doesn't depend on
> the internal labels of ALTERNATIVE().  Something like:
>     asm volatile("1:" ALTERNATIVE_3(XSAVE, \
>     ...
>      _ASM_EXTABLE_TYPE_REG(1b, 3b, EX_TYPE_EFAULT_REG, %[err]) \
>     ...

Thanks for the cool idea - that use of the asm label outside of the
macro was really nasty but I didn't think of that. Cool.

And yap, it looks good:

------
#APP
# 188 "arch/x86/kernel/fpu/xstate.h" 1
	1: # ALT: oldinstr

<--- the outer label 1:

771:
	# ALT: oldinstr
771:
	# ALT: oldinstr
771:
	.byte 0x48, 0x0f,0xae,0x27
772:
# ALT: padding
.skip -(((775f-774f)-(772b-771b)) > 0) * ((775f-774f)-(772b-771b)),0x90

...

triple-alternative gunk

...

775:
.popsection

xor %edi, %edi	# err
3:
 .pushsection "__ex_table","a"
------

Yap, and boots in the guest.

I'll fold in the below:

diff --git a/arch/x86/kernel/fpu/xstate.h b/arch/x86/kernel/fpu/xstate.h
index 70903c12a911..2ee0b9c53dcc 100644
--- a/arch/x86/kernel/fpu/xstate.h
+++ b/arch/x86/kernel/fpu/xstate.h
@@ -106,21 +106,17 @@ static inline u64 xfeatures_mask_independent(void)
  * Otherwise, if XSAVEOPT is enabled, XSAVEOPT replaces XSAVE because XSAVEOPT
  * supports modified optimization which is not supported by XSAVE.
  *
- * We use XSAVE as a fallback.
- *
- * The 771 label is defined in the ALTERNATIVE* macros as the address of the
- * original instruction which gets replaced. We need to use it here as the
- * address of the instruction where we might get an exception at.
+ * Use XSAVE as a fallback.
  */
 #define XSTATE_XSAVE(st, lmask, hmask, err)				\
-	asm volatile(ALTERNATIVE_3(XSAVE,				\
+	asm volatile("1: " ALTERNATIVE_3(XSAVE,				\
 				   XSAVEOPT, X86_FEATURE_XSAVEOPT,	\
 				   XSAVEC,   X86_FEATURE_XSAVEC,	\
 				   XSAVES,   X86_FEATURE_XSAVES)	\
 		     "\n"						\
 		     "xor %[err], %[err]\n"				\
 		     "3:\n"						\
-		     _ASM_EXTABLE_TYPE_REG(771b, 3b, EX_TYPE_EFAULT_REG, %[err]) \
+		     _ASM_EXTABLE_TYPE_REG(1b, 3b, EX_TYPE_EFAULT_REG, %[err]) \
 		     : [err] "=r" (err)					\
 		     : "D" (st), "m" (*st), "a" (lmask), "d" (hmask)	\
 		     : "memory")
@@ -130,11 +126,11 @@ static inline u64 xfeatures_mask_independent(void)
  * XSAVE area format.
  */
 #define XSTATE_XRESTORE(st, lmask, hmask)				\
-	asm volatile(ALTERNATIVE(XRSTOR,				\
+	asm volatile("1: " ALTERNATIVE(XRSTOR,				\
 				 XRSTORS, X86_FEATURE_XSAVES)		\
 		     "\n"						\
 		     "3:\n"						\
-		     _ASM_EXTABLE_TYPE(771b, 3b, EX_TYPE_FPU_RESTORE)	\
+		     _ASM_EXTABLE_TYPE(1b, 3b, EX_TYPE_FPU_RESTORE)	\
 		     :							\
 		     : "D" (st), "m" (*st), "a" (lmask), "d" (hmask)	\
 		     : "memory")

-- 
Regards/Gruss,
    Boris.

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