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

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Message-ID: <20250728203809.7bebf01c@pumpkin>
Date: Mon, 28 Jul 2025 20:38:09 +0100
From: David Laight <david.laight.linux@...il.com>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, Andy Lutomirski
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Subject: Re: [PATCHv9 02/16] x86/alternatives: Disable LASS when patching
 kernel alternatives

On Mon, 28 Jul 2025 12:28:33 -0700
"H. Peter Anvin" <hpa@...or.com> wrote:

> On July 28, 2025 12:11:37 PM PDT, David Laight <david.laight.linux@...il.com> wrote:
> >On Mon,  7 Jul 2025 11:03:02 +0300
> >"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com> wrote:
> >  
> >> From: Sohil Mehta <sohil.mehta@...el.com>
> >> 
> >> For patching, the kernel initializes a temporary mm area in the lower
> >> half of the address range. See commit 4fc19708b165 ("x86/alternatives:
> >> Initialize temporary mm for patching").
> >> 
> >> Disable LASS enforcement during patching to avoid triggering a #GP
> >> fault.
> >> 
> >> The objtool warns due to a call to a non-allowed function that exists
> >> outside of the stac/clac guard, or references to any function with a
> >> dynamic function pointer inside the guard. See the Objtool warnings
> >> section #9 in the document tools/objtool/Documentation/objtool.txt.
> >> 
> >> Considering that patching is usually small, replace the memcpy() and
> >> memset() functions in the text poking functions with their open coded
> >> versions.  
> >...
> >
> >Or just write a byte copy loop in C with (eg) barrier() inside it
> >to stop gcc converting it to memcpy().
> >
> >	David  
> 
> Great. It's rep movsb without any of the performance.

And without the massive setup overhead that dominates short copies.
Given the rest of the code I'm sure a byte copy loop won't make
any difference to the overall performance.

	David