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Expand the prefault memory selftest to add a regression test for a KVM bug
where KVM's retry logic would result in (breakable) deadlock due to the
memslot deletion waiting on prefaulting to release SRCU, and prefaulting
waiting on the memslot to fully disappear (KVM uses a two-step process to
delete memslots, and KVM x86 retries page faults if a to-be-deleted, a.k.a.
INVALID, memslot is encountered).
To exercise concurrent memslot remove, spawn a second thread to initiate
memslot removal at roughly the same time as prefaulting. Test memslot
removal for all testcases, i.e. don't limit concurrent removal to only the
success case. There are essentially three prefault scenarios (so far)
that are of interest:
1. Success
2. ENOENT due to no memslot
3. EAGAIN due to INVALID memslot
For all intents and purposes, #1 and #2 are mutually exclusive, or rather,
easier to test via separate testcases since writing to non-existent memory
is trivial. But for #3, making it mutually exclusive with #1 _or_ #2 is
actually more complex than testing memslot removal for all scenarios. The
only requirement to let memslot removal coexist with other scenarios is a
way to guarantee a stable result, e.g. that the "no memslot" test observes
ENOENT, not EAGAIN, for the final checks.
So, rather than make memslot removal mutually exclusive with the ENOENT
scenario, simply restore the memslot and retry prefaulting. For the "no
memslot" case, KVM_PRE_FAULT_MEMORY should be idempotent, i.e. should
always fail with ENOENT regardless of how many times userspace attempts
prefaulting.
Pass in both the base GPA and the offset (instead of the "full" GPA) so
that the worker can recreate the memslot.
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Co-developed-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20250924174255.2141847-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
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