Re: [PATCHv6 00/16] x86: Enable Linear Address Space Separation support

Andrew Cooper
2025-06-20T22:04:57Z

> Most of the cover letter here reads like an Intel whitepaper. That's not
> the worst thing in the world, but I think it glosses over one very
> important point:
>
> Had it been available, LASS alone would have mitigated Meltdown.
>
> Could we say this up front in a prominent place, please?

I'm going to nitpick. :)

Yes, LASS would have made Meltdown a far less major problem than it was,
but I don't think that phrasing is fair.  As I recall, LASS was
literally invented as a "what would have been useful?" exercise in the
wake of Meltdown.

However, a less well known/researched area of Meltdown, which would not
be addressed by LASS, is the ability to pend stores to read-only memory
and proceed with the expectation that they'll success.

Other things which would have helped would have been PKS (and this *was*
asked for ahead of Skylake launching...)

The other important thing about LASS is that it does cut off a whole
class of sidechannels.  This halts definitely-rogue speculation, but is
useful for non-speculative security too.

~Andrew