On Thu, Aug 29, 2019 at 10:30:51AM -0400, Phil Auld wrote: > On Wed, Aug 28, 2019 at 06:01:14PM +0200 Peter Zijlstra wrote: > > On Wed, Aug 28, 2019 at 11:30:34AM -0400, Phil Auld wrote: > > > On Tue, Aug 27, 2019 at 11:50:35PM +0200 Peter Zijlstra wrote: > > > > > > And given MDS, I'm still not entirely convinced it all makes sense. If > > > > it were just L1TF, then yes, but now... > > > > > > I was thinking MDS is really the reason for this. L1TF has mitigations but > > > the only current mitigation for MDS for smt is ... nosmt. > > > > L1TF has no known mitigation that is SMT safe. The moment you have > > something in your L1, the other sibling can read it using L1TF. > > > > The nice thing about L1TF is that only (malicious) guests can exploit > > it, and therefore the synchronizatin context is VMM. And it so happens > > that VMEXITs are 'rare' (and already expensive and thus lots of effort > > has already gone into avoiding them). > > > > If you don't use VMs, you're good and SMT is not a problem. > > > > If you do use VMs (and do/can not trust them), _then_ you need > > core-scheduling; and in that case, the implementation under discussion > > misses things like synchronization on VMEXITs due to interrupts and > > things like that. > > > > But under the assumption that VMs don't generate high scheduling rates, > > it can work. > > > > > The current core scheduler implementation, I believe, still has > > > (theoretical?) > > > holes involving interrupts, once/if those are closed it may be even less > > > attractive. > > > > No; so MDS leaks anything the other sibling (currently) does, this makes > > _any_ privilidge boundary a synchronization context. > > > > Worse still, the exploit doesn't require a VM at all, any other task can > > get to it. > > > > That means you get to sync the siblings on lovely things like system > > call entry and exit, along with VMM and anything else that one would > > consider a privilidge boundary. Now, system calls are not rare, they > > are really quite common in fact. Trying to sync up siblings at the rate > > of system calls is utter madness. > > > > So under MDS, SMT is completely hosed. If you use VMs exclusively, then > > it _might_ work because a 'pure' host doesn't schedule that often > > (maybe, same assumption as for L1TF). > > > > Now, there have been proposals of moving the privilidge boundary further > > into the kernel. Just like PTI exposes the entry stack and code to > > Meltdown, the thinking is, lets expose more. By moving the priv boundary > > the hope is that we can do lots of common system calls without having to > > sync up -- lots of details are 'pending'. > > > Thanks for clarifying. My understanding is (somewhat) less fuzzy now. :) > > I think, though, that you were basically agreeing with me that the current > core scheduler does not close the holes, or am I reading that wrong.
Agreed; the missing bits for L1TF are ugly but doable (I've actually done them before, Tim has that _somewhere_), but I've not seen a 'workable' solution for MDS yet.
