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

Sohil Mehta
2025-06-20T23:10:38Z

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 :)