[MODERATED] Re: [PATCH 5/5] SSB extra v2 5

[Linus Torvalds <torvalds@linux-foundation.org>]

On Wed, 9 May 2018, speck for Dave Hansen wrote:
> 
> Frankly, I think it's mostly a waste of time to do it without having the
> BPF folks deeply involved from day one.  They were not fans of the way
> we (Intel) did Spectre V1 mitigations and I don't expect this to be any
> different.
> 
> They were deeply opposed to LFENCE getting used, for instance.

If the main worry is using the store buffer bypass to basically bypass 
type safety (by loading a pointer that was prevously stored as something 
else), I would assume that it could be done with something much more 
targeted than an lfence. 

But I do suspect that once they see a MSR write on each entry/exit, 
they'll say "we'll do the lfence". You can probably have some simple 
greedy model where you only do an lfence for pointer loads, and only if 
you did a store before, and maybe it won't be too painful.

But yes, we definitely should have Alexei involved. For some reason I 
thought he already was.

              Linus