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

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Message-ID: <cqacx3crogegwyslm25kwcdcezgg2n44lhy3mg5qkka3vgn4xa@lhqsoseyduus>
Date: Thu, 11 Jul 2024 01:33:23 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: 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>, 
	Brijesh Singh <brijesh.singh@....com>, Michael Roth <michael.roth@....com>, 
	Tony Luck <tony.luck@...el.com>, Alexey Kardashevskiy <aik@....com>, 
	Jonathan Corbet <corbet@....net>, Sohil Mehta <sohil.mehta@...el.com>, 
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	Daniel Sneddon <daniel.sneddon@...ux.intel.com>, Kai Huang <kai.huang@...el.com>, 
	Sandipan Das <sandipan.das@....com>, Breno Leitao <leitao@...ian.org>, 
	Rick Edgecombe <rick.p.edgecombe@...el.com>, Yian Chen <yian.chen@...el.com>, 
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	Vegard Nossum <vegard.nossum@...cle.com>, Kees Cook <kees@...nel.org>, Eric Biggers <ebiggers@...gle.com>, 
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	Andrew Morton <akpm@...ux-foundation.org>, Luis Chamberlain <mcgrof@...nel.org>, 
	Yuntao Wang <ytcoode@...il.com>, Rasmus Villemoes <linux@...musvillemoes.dk>, 
	Christophe Leroy <christophe.leroy@...roup.eu>, Tejun Heo <tj@...nel.org>, Changbin Du <changbin.du@...wei.com>, 
	Huang Shijie <shijie@...amperecomputing.com>, Geert Uytterhoeven <geert+renesas@...der.be>, 
	Namhyung Kim <namhyung@...nel.org>, 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 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.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov