The CPU_ON PSCI call requires careful coordination between vCPUs in KVM,
as it allows callers to send a payload (pc, context id) to another vCPU
to start execution. There are a couple of races in the handling of
CPU_ON:
- KVM uses the kvm->lock to serialize the write-side of a vCPU's reset
state. However, kvm_vcpu_reset() doesn't take the lock on the
read-size, meaning the vCPU could be reset with interleaved state
from two separate CPU_ON calls.
- If a targeted vCPU never enters the guest again (say, the VMM was
getting ready to migrate), then the reset payload is never actually
folded in to the vCPU's registers. Despite this, the calling vCPU has
already made the target runnable. Migrating the target vCPU at this
time will result in execution from its old PC, not execution coming
out of the reset state at the requested address.
Patch 1 addresses the read-side race in KVM's CPU_ON implementation.
Patch 2 fixes the KVM/VMM race by resetting a vCPU (if requested)
whenever the VMM tries to read out its registers. Gross, but it avoids
exposing the vcpu_reset_state structure through some other UAPI. That is
undesirable, as we really are only trying to paper over the
implementation details of PSCI in KVM.
Patch 3 is unrelated, and is based on my own reading of the PSCI
specification. In short, if you invoke PSCI_ON from AArch64, then you
must set the Aff3 bits. This is impossible if you use the 32 bit
function, since the arguments are only 32 bits. Just return
INVALID_PARAMS to the guest in this case.
This series cleanly applies to kvm-arm/next at the following commit:
ae280335cdb5 ("Merge branch kvm-arm64/mmu/el2-tracking into kvmarm-master/next")
The series was tested with the included KVM selftest on an Ampere Mt.
Jade system. Broken behavior was verified using the same test on
kvm-arm/next, sans this series.
Oliver Upton (4):
KVM: arm64: Fix read-side race on updates to vcpu reset state
KVM: arm64: Handle PSCI resets before userspace touches vCPU state
KVM: arm64: Enforce reserved bits for PSCI target affinities
selftests: KVM: Introduce psci_cpu_on_test
arch/arm64/kvm/arm.c | 9 ++
arch/arm64/kvm/psci.c | 20 ++-
arch/arm64/kvm/reset.c | 16 ++-
tools/testing/selftests/kvm/.gitignore | 1 +
tools/testing/selftests/kvm/Makefile | 1 +
.../selftests/kvm/aarch64/psci_cpu_on_test.c | 121 ++++++++++++++++++
.../selftests/kvm/include/aarch64/processor.h | 3 +
7 files changed, 162 insertions(+), 9 deletions(-)
create mode 100644 tools/testing/selftests/kvm/aarch64/psci_cpu_on_test.c
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2.33.0.rc1.237.g0d66db33f3-goog
