Re: [PATCHv9 02/16] x86/alternatives: Disable LASS when patching kernel alternatives

[Sohil Mehta]

On 7/9/2025 9:58 AM, Dave Hansen wrote:

>> +	 * Avoid using memcpy() here. Instead, open code it.
>> +	 */
>> +	asm volatile("rep movsb"
>> +		     : "+D" (dst), "+S" (src), "+c" (len) : : "memory");
>> +
>> +	lass_clac();
>>  }
> 
> This didn't turn out great. At the _very_ least, we could have a:
> 
> 	inline_memcpy_i_really_mean_it()
> 

It looks like we should go back to __inline_memcpy()/_memset()
implementation that PeterZ had initially proposed. It seems to fit all
the requirements, right? Patch attached.

https://lore.kernel.org/lkml/20241028160917.1380714-3-alexander.shishkin@xxxxxxxxxxxxxxx/

> with the rep mov. Or even a #define if we were super paranoid the
> compiler is out to get us.
> 
> But _actually_ open-coding inline assembly is far too ugly to live.
> From eb3b45b377df90d3b367e2b3fddfff1a72624a4e Mon Sep 17 00:00:00 2001
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Mon, 28 Oct 2024 18:07:50 +0200
Subject: [PATCH] x86/asm: Introduce inline memcpy and memset

Provide inline memcpy and memset functions that can be used instead of
the GCC builtins whenever necessary.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Sohil Mehta <sohil.mehta@xxxxxxxxx>
---
 arch/x86/include/asm/string.h | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/arch/x86/include/asm/string.h b/arch/x86/include/asm/string.h
index c3c2c1914d65..9cb5aae7fba9 100644
--- a/arch/x86/include/asm/string.h
+++ b/arch/x86/include/asm/string.h
@@ -1,6 +1,32 @@
 /* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_STRING_H
+#define _ASM_X86_STRING_H
+
 #ifdef CONFIG_X86_32
 # include <asm/string_32.h>
 #else
 # include <asm/string_64.h>
 #endif
+
+static __always_inline void *__inline_memcpy(void *to, const void *from, size_t len)
+{
+	void *ret = to;
+
+	asm volatile("rep movsb"
+		     : "+D" (to), "+S" (from), "+c" (len)
+		     : : "memory");
+	return ret;
+}
+
+static __always_inline void *__inline_memset(void *s, int v, size_t n)
+{
+	void *ret = s;
+
+	asm volatile("rep stosb"
+		     : "+D" (s), "+c" (n)
+		     : "a" ((uint8_t)v)
+		     : "memory");
+	return ret;
+}
+
+#endif /* _ASM_X86_STRING_H */
-- 
2.43.0