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Message-ID: <20250706101342.069b5068@pumpkin> Date: Sun, 6 Jul 2025 10:13:42 +0100 From: David Laight <david.laight.linux@...il.com> To: Dave Hansen <dave.hansen@...el.com> Cc: "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 <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>, Alexander Shishkin <alexander.shishkin@...ux.intel.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>, 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, linux-mm@...ck.org Subject: Re: [PATCHv8 02/17] x86/asm: Introduce inline memcpy and memset On Thu, 3 Jul 2025 10:13:44 -0700 Dave Hansen <dave.hansen@...el.com> wrote: > On 7/1/25 02:58, Kirill A. Shutemov wrote: > > Extract memcpy and memset functions from copy_user_generic() and > > __clear_user(). > > > > They can be used as inline memcpy and memset instead of the GCC builtins > > whenever necessary. LASS requires them to handle text_poke. > > Why are we messing with the normal user copy functions? Code reuse is > great, but as you're discovering, the user copy code is highly > specialized and not that easy to reuse for other things. > > Don't we just need a dirt simple chunk of code that does (logically): > > stac(); > asm("rep stosq..."); > clac(); > > Performance doesn't matter for text poking, right? It could be stosq or > anything else that you can inline. It could be a for() loop for all I > care as long as the compiler doesn't transform it into some out-of-line > memset. Right? > It doesn't even really matter if there is an out-of-line memset. All you need to do is 'teach' objtool it isn't a problem. Is this for the boot-time asm-alternatives? In that case I wonder why a 'low' address is being used? With LASS enabled using a low address on a life kernel would make it harder for another cpu to leverage the writable code page, but that isn't a requirement of LASS. If it is being used for later instruction patching you need the very careful instruction sequences and cpu synchronisation. In that case I suspect you need to add conditional stac/clac to the existing patching code (and teach objtool it is all ok). David
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