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Message-ID: <20240711081459.GE4587@noisy.programming.kicks-ass.net>
Date: Thu, 11 Jul 2024 10:14:59 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Borislav Petkov <bp@...en8.de>,
	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>, 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>, Ingo Molnar <mingo@...nel.org>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
	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>,
	Alexei Starovoitov <ast@...nel.org>, Hou Tao <houtao1@...wei.com>,
	Juergen Gross <jgross@...e.com>,
	Vegard Nossum <vegard.nossum@...cle.com>,
	Kees Cook <kees@...nel.org>, Eric Biggers <ebiggers@...gle.com>,
	Jason Gunthorpe <jgg@...pe.ca>,
	"Masami Hiramatsu (Google)" <mhiramat@...nel.org>,
	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 Thu, Jul 11, 2024 at 01:33:23AM +0300, 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.

The one where performance might matter is text_poke_early(), the full
fat one is comically slow as you point out.

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