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

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Message-ID: <e2765d39-5f44-4c60-b37f-c252c8a98f3d@intel.com>
Date: Thu, 24 Jul 2025 19:35:56 -0700
From: Sohil Mehta <sohil.mehta@...el.com>
To: Dave Hansen <dave.hansen@...el.com>, "Kirill A. Shutemov"
	<kirill.shutemov@...ux.intel.com>, Andy Lutomirski <luto@...nel.org>, "Thomas
 Gleixner" <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "Borislav
 Petkov" <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
	<x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>, Peter Zijlstra
	<peterz@...radead.org>, Ard Biesheuvel <ardb@...nel.org>, "Paul E. McKenney"
	<paulmck@...nel.org>, Josh Poimboeuf <jpoimboe@...nel.org>, Xiongwei Song
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 (IBM)" <rppt@...nel.org>, Brijesh Singh <brijesh.singh@....com>, Michael Roth
	<michael.roth@....com>, Tony Luck <tony.luck@...el.com>, Alexey Kardashevskiy
	<aik@....com>, Alexander Shishkin <alexander.shishkin@...ux.intel.com>
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 Gupta" <pawan.kumar.gupta@...ux.intel.com>, Daniel Sneddon
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	<kas@...nel.org>
Subject: Re: [PATCHv9 02/16] x86/alternatives: Disable LASS when patching
 kernel alternatives

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@linux.intel.com/

> 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.
> 
View attachment "x86-asm-Introduce-inline-memcpy-and-memset.patch" of type "text/plain" (1419 bytes)