If a 32-bit guest accesses MMIO using a 16-bit Thumb-2 instruction that is reported to the hypervisor without a valid syndrom (for example, because of the addressing mode), then we may hand off the fault to userspace. When resuming the guest, we unconditionally advance the PC by 4 bytes, since ESR_EL2.IL is always 1 for data aborts generated without a valid syndrome. This is a bit rubbish, but it's also difficult to see how we can fix it without potentially introducing regressions in userspace MMIO fault handling.
Update the comment when skipping a guest MMIO access instruction so that this corner case is at least written down. Cc: Marc Zyngier <[email protected]> Cc: Quentin Perret <[email protected]> Signed-off-by: Will Deacon <[email protected]> --- arch/arm64/kvm/mmio.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/arch/arm64/kvm/mmio.c b/arch/arm64/kvm/mmio.c index 4e0366759726..b54ea5aa6c06 100644 --- a/arch/arm64/kvm/mmio.c +++ b/arch/arm64/kvm/mmio.c @@ -113,6 +113,13 @@ int kvm_handle_mmio_return(struct kvm_vcpu *vcpu, struct kvm_run *run) /* * The MMIO instruction is emulated and should not be re-executed * in the guest. + * + * Note: If user space handled the emulation because the abort + * symdrome information was not valid (ISV set in the ESR), then + * this will assume that the faulting instruction was 32-bit. + * If the faulting instruction was a 16-bit Thumb instruction, + * then userspace would need to rewind the PC by 2 bytes prior to + * resuming the vCPU (yuck!). */ kvm_skip_instr(vcpu, kvm_vcpu_trap_il_is32bit(vcpu)); -- 2.28.0.rc0.142.g3c755180ce-goog _______________________________________________ kvmarm mailing list [email protected] https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
