On Wed, Jul 10, 2024, James Houghton wrote: > On Tue, Jul 9, 2024 at 10:49 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > > > On Mon, Jul 08, 2024, James Houghton wrote: > > > On Fri, Jun 28, 2024 at 7:38 PM James Houghton <jthoughton@xxxxxxxxxx> wrote: > > > > > > > > On Mon, Jun 17, 2024 at 11:37 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > > I still don't think we should get rid of the WAS_FAST stuff. > > > > I do :-) > > > > > The assumption that the L1 VM will almost never share pages between L2 > > > VMs is questionable. The real question becomes: do we care to have > > > accurate age information for this case? I think so. > > > > I think you're conflating two different things. WAS_FAST isn't about accuracy, > > it's about supporting lookaround in conditionally fast secondary MMUs. > > > > Accuracy only comes into play when we're talking about the last-minute check, > > which, IIUC, has nothing to do with WAS_FAST because any potential lookaround has > > already been performed. > > Sorry, I thought you meant: have the MMU notifier only ever be > lockless (when tdp_mmu_enabled), and just return a potentially wrong > result in the unlikely case that L1 is sharing pages between L2s. > > I think it's totally fine to just drop WAS_FAST. So then we can either > do look-around (1) always, or (2) only when there is a secondary MMU > with has_fast_aging. (2) is pretty simple, I'll just do that. > > We can add some shadow MMU lockless support later to make the > look-around not as useless for the nested TDP case. ... > > Adding the locking isn't actually all that difficult, with the *huge* caveat that > > the below patch is compile-tested only. The vast majority of the churn is to make > > it so existing code ignores the new KVM_RMAP_LOCKED bit. > > This is very interesting, thanks for laying out how this could be > done. I don't want to hold this series up on getting the details of > the shadow MMU lockless walk exactly right. :) ... > 1. Drop the WAS_FAST complexity. > 2. Add a function like mm_has_fast_aging_notifiers(), use that to > determine if we should be doing look-around. I would prefer a flag over a function. Long-term, if my pseudo-lockless rmap idea pans out, KVM can set the flag during VM creation. Until then, KVM can set the flag during creation and then toggle it in (un)account_shadowed(). Races will be possible, but they should be extremely rare and quite benign, all things considered.
