Problem =======
When host APEI is unable to claim synchronous external abort (SEA) during stage-2 guest abort, today KVM directly injects an async SError into the VCPU then resumes it. The injected SError usually results in unpleasant guest kernel panic.
One of the major situation of guest SEA is when VCPU consumes recoverable uncorrected memory error (UER), which is not uncommon at all in modern datacenter servers with large amounts of physical memory. Although SError and guest panic is sufficient to stop the propagation of corrupted memory there is still room to recover from memory UER in a more graceful manner.
Proposed Solution =================
Alternatively KVM can replay the SEA to the faulting VCPU, via existing KVM_SET_VCPU_EVENTS API. If the memory poison consumption or the fault that cause SEA is not from guest kernel, the blast radius can be limited to the consuming or faulting guest userspace process, so the VM can keep running.
In addition, instead of doing under the hood without involving userspace, there are benefits to redirect the SEA to VMM:
- VM customers care about the disruptions caused by memory errors, and VMM usually has the responsibility to start the process of notifying the customers of memory error events in their VMs. For example some cloud provider emits a critical log in their observability UI [1], and provides playbook for customers on how to mitigate disruptions to their workloads.
- VMM can protect future memory error consumption or faults by unmapping the poisoned pages from stage-2 page table with KVM userfault [2], which is more performant than splitting the memslot that contains the poisoned guest pages.
- VMM can keep track SEA events in the VM. When VMM thinks the status on the host or the VM is bad enough, e.g. number of distinct SEAs exceeds a threshold, it can restart the VM on another healthy host.
- Behavior parity with x86 architecture. When machine check exception (MCE) is caused by VCPU, kernel or KVM signals userspace SIGBUS to let VMM either recover from the MCE, or terminate itself with VM. The prior RFC proposes to implement SIGBUS on arm64 as well, but Marc preferred VCPU exit over signal [3]. However, implementation aside, returning SEA to VMM is on par with returning MCE to VMM.
Once SEA is redirected to VMM, among other actions, VMM is encouraged to inject external aborts into the faulting VCPU, which is already supported by KVM on arm64, although not fully supported by KVM_SET_VCPU_EVENTS but complemented in this patchset.
New UAPIs =========
This patchset introduces following userspace-visiable changes to empower VMM to control what happens next for guest SEA:
- KVM_CAP_ARM_SEA_TO_USER. If userspace enables this new capability at VM creation, KVM will not inject SError while taking SEA, but VM exit to userspace.
- KVM_EXIT_ARM_SEA. This is the VM exit reason VMM gets. The details about the SEA is provided in arm_sea as much as possible, including ESR value at EL2, if guest virtual and physical addresses (GPA and GVA) are available and the values if available.
- KVM_CAP_ARM_INJECT_EXT_IABT. VMM today can inject external data abort to VCPU via KVM_SET_VCPU_EVENTS API. However, in case of instruction abort, VMM cannot inject it via KVM_SET_VCPU_EVENTS. KVM_CAP_ARM_INJECT_EXT_IABT is just a natural extend to KVM_CAP_ARM_INJECT_EXT_DABT that tells VMM KVM_SET_VCPU_EVENTS now supports external instruction abort.
Patchset utilizes commit 26fbdf369227 ("KVM: arm64: Don't translate FAR if invalid/unsafe") from [4], available already in kvmarm/next. [4] makes KVM safely do address translation for HPFAR_EL2, including at the event of SEA, and indicate if HPFAR_EL2 is valid in NS bit. This patchset depends on [4] to tell userspace if GPA is valid and its value if valid.
Patchset is based on commit 68ec8b4e84446 ("Merge branch kvm-arm64/pkvm-6.16 into kvmarm-master/next")
[1] https://cloud.google.com/solutions/sap/docs/manage-host-errors [2] https://lpc.events/event/18/contributions/1757/attachments/1442/3073/LPC_%20... [3] https://lore.kernel.org/kvm/86pljbqqh0.wl-maz@kernel.org [4] https://lore.kernel.org/all/174369514508.3034362.13165690020799838042.b4-ty@...
Jiaqi Yan (5): KVM: arm64: VM exit to userspace to handle SEA KVM: arm64: Set FnV for VCPU when FAR_EL2 is invalid KVM: selftests: Test for KVM_EXIT_ARM_SEA and KVM_CAP_ARM_SEA_TO_USER KVM: selftests: Test for KVM_CAP_INJECT_EXT_IABT Documentation: kvm: new uAPI for handling SEA
Raghavendra Rao Ananta (1): KVM: arm64: Allow userspace to inject external instruction aborts
Documentation/virt/kvm/api.rst | 120 ++++++- arch/arm64/include/asm/kvm_emulate.h | 12 + arch/arm64/include/asm/kvm_host.h | 8 + arch/arm64/include/asm/kvm_ras.h | 21 +- arch/arm64/include/uapi/asm/kvm.h | 3 +- arch/arm64/kvm/Makefile | 3 +- arch/arm64/kvm/arm.c | 6 + arch/arm64/kvm/guest.c | 13 +- arch/arm64/kvm/inject_fault.c | 3 + arch/arm64/kvm/kvm_ras.c | 54 +++ arch/arm64/kvm/mmu.c | 12 +- include/uapi/linux/kvm.h | 12 + tools/arch/arm64/include/uapi/asm/kvm.h | 3 +- tools/testing/selftests/kvm/Makefile.kvm | 2 + .../testing/selftests/kvm/arm64/inject_iabt.c | 100 ++++++ .../testing/selftests/kvm/arm64/sea_to_user.c | 324 ++++++++++++++++++ tools/testing/selftests/kvm/lib/kvm_util.c | 1 + 17 files changed, 654 insertions(+), 43 deletions(-) create mode 100644 arch/arm64/kvm/kvm_ras.c create mode 100644 tools/testing/selftests/kvm/arm64/inject_iabt.c create mode 100644 tools/testing/selftests/kvm/arm64/sea_to_user.c