linux-kernel - Re: [PATCHv7 03/16] x86/alternatives: Disable LASS when patching kernel alternatives

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Message-ID: <oufhbjxqhdma2oe7g3j7xfbqtkazm5d2fzi7im3xqfwysnr5mo@rzfvqa6bdfmk>
Date: Fri, 27 Jun 2025 13:27:41 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Peter Zijlstra <peterz@...radead.org>, 
	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>, 
	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>, 
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	linux-mm@...ck.org
Subject: Re: [PATCHv7 03/16] x86/alternatives: Disable LASS when patching
 kernel alternatives

On Thu, Jun 26, 2025 at 07:18:59AM -0700, Dave Hansen wrote:
> On 6/26/25 06:49, Peter Zijlstra wrote:
> >> +static __always_inline void lass_enable_enforcement(void)
> >> +{
> >> +	alternative("", "clac", X86_FEATURE_LASS);
> >> +}
> >> +
> >> +static __always_inline void lass_disable_enforcement(void)
> >> +{
> >> +	alternative("", "stac", X86_FEATURE_LASS);
> >> +}
> > Much hate for this naming. WTH was wrong with lass_{clac,stac}()?
> > 
> > We're not calling those other functions smap_{en,dis}able_enforcement()
> > either (and please don't take that as a suggestion, its terrible
> > naming).
> 
> It was a response to a comment from Sohil about the delta between
> lass_{cl,st}ac() and plain {cl,st}ac() being subtle. They are subtle,
> but I don't think it's fixable with naming.
> 
> There are lots of crazy gymnastics we could do. But there are so few
> sites where AC is twiddled for LASS that I don't think it's worth it.
> 
> Let's just use the lass_{cl,st}ac() and comment both variants. First,
> the existing stac()/clac():
> 
> /*
>  * Use these when accessing userspace (_PAGE_USER)
>  * mappings, regardless of location.
>  */
> 
> and the new ones:
> 
> /*
>  * Use these when accessing kernel mappings (!_PAGE_USER)
>  * in the lower half of the address space.
>  */
> 
> Any objections to doing that?

Looks good. Will update the patch.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov