linux-kernel - Re: [PATCH v4 03/16] x86/alternatives: Disable LASS when patching kernel alternatives

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Message-ID: <7bbf9cae-6392-47a4-906c-7c27b1b1223d@intel.com>
Date: Wed, 10 Jul 2024 16:05:31 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 Borislav Petkov <bp@...en8.de>
Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Andy Lutomirski <luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, 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 <xiongwei.song@...driver.com>,
 Xin Li <xin3.li@...el.com>, "Mike Rapoport (IBM)" <rppt@...nel.org>,
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 Arnaldo Carvalho de Melo <acme@...hat.com>, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org
Subject: Re: [PATCH v4 03/16] x86/alternatives: Disable LASS when patching
 kernel alternatives

On 7/10/24 15:33, Kirill A. Shutemov wrote:
> On Wed, Jul 10, 2024 at 07:18:36PM +0200, Borislav Petkov wrote:
>> On Wed, Jul 10, 2024 at 07:06:39PM +0300, Alexander Shishkin wrote:
>>>  static void text_poke_memcpy(void *dst, const void *src, size_t len)
>>>  {
>>> -	memcpy(dst, src, len);
>>> +	stac();
>>> +	__inline_memcpy(dst, src, len);
>>> +	clac();
>> I think you need LASS-specific stac()/clac() or an alternative_2 or so. You
>> can't cause that perf penalty on !LASS machines.
> Hm. Do we have text_poke() in hot path?
> 
> Even if we do, I doubt flipping AC flag would make any performance
> difference in context of all locking and TLB flushing we do in this
> codepath.

Yeah, I'm also wondering how much this would matter for performance.

But, I'm 100% sure that we want to distinguish a LASS-necessitated
stac()/clac() from a SMAP-necessitated one somehow.