We talked about shared (faultable) vs. private (unfaultable), and how it
would interact with the directmap patches here.
As discussed, having private (unfaultable) memory with the direct-map
removed and shared (faultable) memory with the direct-mapping can make
sense for non-TDX/AMD-SEV/... non-CoCo use cases. Not sure about CoCo,
the discussion here seems to indicate that it might currently not be
required.
So one thing we could do is that shared (faultable) will have a direct
mapping and be gup-able and private (unfaultable) memory will not have a
direct mapping and is, by design, not gup-able.>
Maybe it could make sense to not have a direct map for all guest_memfd
memory, making it behave like secretmem (and it would be easy to
implement)? But I'm not sure if that is really desirable in VM context.
This would work for us (in this scenario, the swiotlb areas would be
"traditional" memory, e.g. set to shared via mem attributes instead of
"shared" inside KVM), it's kinda what I had prototyped in my v1 of this
series (well, we'd need to figure out how to get the mappings of gmem
back into KVM, since in this setup, short-circuiting it into
userspace_addr wouldn't work, unless we banish swiotlb into a different
memslot altogether somehow).
Right.
But I don't think it'd work for pKVM, iirc
they need GUP on gmem, and also want direct map removal (... but maybe,
the gmem VMA for non-CoCo usecase and the gmem VMA for pKVM could be
behave differently? non-CoCo gets essentially memfd_secret, pKVM gets
GUP+no faults of private mem).
Good question. So far my perception was that the directmap removal on
"private/unfaultable" would be sufficient.
Having a mixture of "has directmap" and "has no directmap" for shared
(faultable) memory should not be done. Similarly, private memory really
should stay "unfaultable".
You've convinced me that having both GUP-able and non GUP-able
memory in the same VMA will be tricky. However, I'm less convinced on
why private memory should stay unfaultable; only that it shouldn't be
faultable into a VMA that also allows GUP. Can we have two VMAs? One
that disallows GUP, but allows userspace access to shared and private,
and one that allows GUP, but disallows accessing private memory? Maybe
via some `PROT_NOGUP` flag to `mmap`? I guess this is a slightly
different spin of the above idea.
What we are trying to achieve is making guest_memfd not behave
completely different on that level for different "types" of VMs. So one
of the goals should be to try to unify it as much as possible.
shared -> faultable: GUP-able
private -> unfaultable: unGUP-able
And it makes sense, because a lot of future work will rely on some
important properties: for example, if private memory cannot be faulted
in + GUPed, core-MM will never have obtained valid references to such a
page. There is no need to split large folios into smaller ones for
tracking purposes; there is no need to maintain per-page refcounts and
pincounts ...
It doesn't mean that we cannot consider it if really required, but there
really has to be a strong case for it, because it will all get really messy.
For example, one issue is that a folio only has a single mapping
(folio->mapping), and that is used in the GUP-fast path (no VMA) to
determine whether GUP-fast is allowed or not.
So you'd have to force everything through GUP-slow, where you could
consider VMA properties :( It sounds quite suboptimal.
I don't think multiple VMAs are what we really want. See below.
I think one of the points raised during the bi-weekly call was that
using a viommu/swiotlb might be the right call, such that all memory can
be considered private (unfaultable) that is not explicitly
shared/expected to be modified by the hypervisor (-> faultable, ->
GUP-able).
Further, I think Sean had some good points why we should explore that
direction, but I recall that there were some issue to be sorted out
(interpreted instructions requiring direct map when accessing "private"
memory?), not sure if that is already working/can be made working in KVM.
Yeah, the big one is MMIO instruction emulation on x86, which does guest
page table walks and instruction fetch (and particularly the latter
cannot be known ahead-of-time by the guest, aka cannot be explicitly
"shared"). That's what the majority of my v2 series was about. For
traditional memslots, KVM handles these via get_user and friends, but if
we don't have a VMA that allows faulting all of gmem, then that's
impossible, and we're in "temporarily restore direct map" land. Which
comes with significantly performance penalties due to TLB flushes.
Agreed.
> >> What's your opinion after the call and the next step for use cases
like
you have in mind (IIRC firecracker, which wants to not have the
direct-map for guest memory where it can be avoided)?
Yea, the usecase is for Firecracker to not have direct map entries for
guest memory, unless needed for I/O (-> swiotlb).
As for next steps, let's determine once and for all if we can do the
KVM-internal guest memory accesses for MMIO emulation through userspace
mappings (although if we can't I'll have some serious soul-searching to
do, because all other solutions we talked about so far also have fairly
big drawbacks; on-demand direct map reinsertion has terrible
performance
So IIUC, KVM would have to access "unfaultable" guest_memfd memory using
fd+offset, and that's problematic because "no-directmap".
So you'd have to map+unmap the directmap repeatedly, and still expose it
temporarily in the direct map to others. I see how that is undesirable,
even when trying to cache hotspots (partly destroying the purpose of the
directmap removal).
Would a per-MM kernel mapping of these pages work, so KVM can access them?
It sounds a bit like what is required for clean per-MM allocations [1]:
establish a per-MM kernel mapping of (selected?) pages. Not necessarily
all of them.
Yes, we'd be avoiding VMAs, GUP, mapcounts, pincounts and everything
involved with ordinary user mappings for these private/unfaultable
thingies. Just like as discussed in, and similar to [1].
Just throwing it out there, maybe we really want to avoid the directmap
(keep it unmapped) and maintain a per-mm mapping for a bunch of folios
that can be easily removed when required by guest_memfd (ftruncate,
conversion private->shared) on request.
[1] https://lore.kernel.org/all/20240911143421.85612-1-faresx@xxxxxxxxx/T/#u
--
Cheers,
David / dhildenb
