linux-kernel - Re: [PATCHv6 01/16] x86/cpu: Enumerate the LASS feature bits

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Message-ID: <eaae7338-df88-4fc5-953e-3e0584bd5380@intel.com>
Date: Fri, 20 Jun 2025 16:10:17 -0700
From: Sohil Mehta <sohil.mehta@...el.com>
To: Dave Hansen <dave.hansen@...el.com>, "Kirill A. Shutemov"
	<kirill.shutemov@...ux.intel.com>, Dave Hansen <dave.hansen@...ux.intel.com>
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Subject: Re: [PATCHv6 01/16] x86/cpu: Enumerate the LASS feature bits

On 6/20/2025 11:24 AM, Dave Hansen wrote:
> On 6/20/25 11:14, Sohil Mehta wrote:
>> @Dave Hansen, you had suggested separating out the SMAP/LASS AC toggle
>> functions. But, the difference in usage between both of them seems very
>> subtle. Could this be easily misused?
> 
> Logically there are two completely different things:
> 
> 	1. Touching userspace
> 	2. Touching the lower half of the address space
> 
> If it's only userspace in the lower half of the address space, then
> there's no controversy. But the problem obviously occurs when you want
> to touch kernel mappings in the lower half of the address space.
> 

Makes sense. If we decide to go this way, we should capture some of this
thinking into the kernel comments. I find the current wording very
ambiguous.

> I want to preserve the "stac/clas" meaning as just "touch userspace".
> 
> The new functions should be for "touch the lower half of the address
> space" alone. Maybe it should be:
> 
> 	lass_disable_enforcement()
> 	lass_enable_enforcement()
> 
> The only downside of not having stac/clac in the names is that it's not
> obvious that they have an impact on SMAP because they're named to be
> LASS-only. 

Yeah, the reverse is also true, where the SMAP specific clac()/stac()
toggle LASS enforcement :)