On 13.10.22 09:43, Emanuele Giuseppe Esposito wrote:
Am 29/09/2022 um 23:39 schrieb Sean Christopherson:
If we really want to provide a better experience for userspace, why not provide
more primitives to address those specific use cases? E.g. fix KVM's historic wart
of disallowing toggling of KVM_MEM_READONLY, and then add one or more ioctls to:
- Merge 2+ memory regions that are contiguous in GPA and HVA
- Split a memory region into 2+ memory regions
- Truncate a memory region
- Grow a memory region
I looked again at the code and specifically the use case that triggers
the crash in bugzilla. I *think* (correct me if I am wrong), that the
only operation that QEMU performs right now is "grow/shrink".
I remember that there were BUG reports where we'd actually split and run
into that problem. Just don't have them at hand. I think they happened
during early boot when the OS re-configured some PCI thingies.
So *if* we want to go this way, we could start with a simple grow/shrink
API.
Even though we need to consider that this could bring additional
complexity in QEMU. Currently, DELETE+CREATE (grow/shrink) is not
performed one after the other (in my innocent fantasy I was expecting to
find 2 subsequent ioctls in the code), but in QEMU's
address_space_set_flatview(), which seems to first remove all regions
and then add them when changing flatviews.
address_space_update_topology_pass(as, old_view2, new_view, adding=false);
address_space_update_topology_pass(as, old_view2, new_view, adding=true);
I don't think we can change this, as other listeners also rely on such
ordering, but we can still batch all callback requests like I currently
do and process them in kvm_commit(), figuring there which operation is
which.
In other words, we would have something like:
region_del() --> DELETE memslot X -> add it to the queue of operations
region_del() --> DELETE memslot Y -> add it to the queue of operations
region_add() --> CREATE memslot X (size doubled) -> add it to the queue
of operations
region_add() --> CREATE memslot Y (size halved) -> add it to the queue
of operations
...
commit() --> scan QUEUE and figure what to do -> GROW X (+size), SHRINK
Y (-size) -> issue 2 ioctls, GROW and SHRINK.
I communicated resizes (region_resize()) to the notifier in patch #3 of
https://lore.kernel.org/qemu-devel/20200312161217.3590-1-david@xxxxxxxxxx/
You could use that independently of the remainder. It should be less
controversial ;)
But I think it only tackles part of the more generic problem (split/merge).
That would probably require more KVM code overall, but each operation would be more
tightly bounded and thus simpler to define. And I think more precise APIs would
provide other benefits, e.g. growing a region wouldn't require first deleting the
current region, and so could avoid zapping SPTEs and destroying metadata. Merge,
split, and truncate would have similar benefits, though they might be more
difficult to realize in practice.
So essentially grow would not require INVALIDATE. Makes sense, but would
it work also with shrink? I guess so, as the memslot is still present
(but shrinked) right?
Paolo, would you be ok with this smaller API? Probably just starting
with grow and shrink first.
I am not against any of the two approaches:
- my approach has the disadvantage that the list could be arbitrarily
long, and it is difficult to rollback the intermediate changes if
something breaks during the request processing (but could be simplified
by making kvm exit or crash).
- Sean approach could potentially provide more burden to the userspace,
as we need to figure which operation is which. Also from my
understanding split and merge won't be really straightforward to
implement, especially in userspace.
David, any concern from userspace prospective on this "CISC" approach?
In contrast to resizes in QEMU that only affect a single memory
region/slot, splitting/merging is harder to factor out and communicate
to a notifier. As an alternative, we could handle it in the commit stage
in the notifier itself, similar to what my prototype does, and figure
out what needs to be done in there and how to call the proper KVM
interface (and which interface to call).
With virtio-mem (in the future) we might see merges of 2 slots into a
single one, by closing a gap in-between them. In "theory" we could
combine some updates into a single transaction. But it won't be 100s ...
I think I'd prefer an API that doesn't require excessive ad-hoc
extensions later once something in QEMU changes.
I think in essence, all we care about is performing atomic changes that
*have to be atomic*, because something we add during a transaction
overlaps with something we remove during a transaction. Not necessarily
all updates in a transaction!
My prototype essentially detects that scenario, but doesn't call new KVM
interfaces to deal with these.
I assume once we take that into consideration, we can mostly assume that
any such atomic updates (only of the regions that really have to be part
of an atomic update) won't involve more than a handful of memory
regions. We could add a sane KVM API limit.
And if we run into that API limit in QEMU, we can print a warning and do
it non-atomically.
--
Thanks,
David / dhildenb
