Re: [PATCH 1/7] x86/cpu: Enumerate LASS CPUID and CR4 bits

[Dave Hansen <dave.hansen@intel.com>]

On 1/10/23 12:14, Sohil Mehta wrote:
> On 1/9/2023 9:51 PM, Yian Chen wrote:
>> LASS (Linear Address Space Separation) is a CPU feature to
>> prevent speculative address access in user/kernel mode.
> 
> Would it be better to say?
> 
> LASS (Linear Address Space Separation) is a security feature that
> intends to prevent unintentional speculative address access across
> user/kernel mode.

It's more than that, though.  The spec actually says this pretty nicely:

> Linear-address space separation (LASS) is an independent mechanism
> that enforces the same mode-based protections as paging but without
> traversing the paging structures. Because the protections enforced by
> LASS are applied before paging, “probes” by malicious software will
> provide no paging-based timing information

So, it's not _just_ that it can prevent some speculative accesses.  It
completely short-circuits paging itself and *ALL* of the baggage that
goes along with paging.

The TLB, mid-level caches, the page walker itself, the data cache
impact...  all of it.  Gone.

*THAT* is the important part here, IMNHO.