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    SubjectRe: [RFC PATCH v3 00/16] Core scheduling v3
    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.


    Cheers,
    Phil

    --

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