I had a conversation with Marc about some of the quirks around the debug architecture on KVM and incorporated some of his suggestions into a series here. Of course, any glaring mistakes/choices made in this series is on me :-) Anyhow: KVM's implementation of the debug architecture is a bit deviant as it stands. For one, KVM handles the OS Lock as RAZ/WI, even though the architecture mandates it. Additionally, KVM advertises more than it can actually support: FEAT_DoubleLock is exposed as implemented to the guest, though OSDLR_EL1 is handled as RAZ/WI too. Only v8.2+ revisions of the debug architecture permit implementations to omit DoubleLock. Fortunately, the delta between v8.0 and v8.2 is entirely focused on external debug, a feature that KVM does not support and likely never will. So, there isn't much of a hurdle to bump KVM's reported DebugVer to v8.2, thereby allowing KVM to omit DoubleLock from ID_AA64DFR0_EL1. Of the remaining bits of external debug visible to the guest, the only additional thing to address is the OSLAR_EL1 issue by simply context switching the host/guest values. Patch 1 changes the way KVM backs OSLSR_EL1 in the sys reg table. Instead of returning a static value from its handler, stash a copy of it in kvm_cpu_context and return that when read. Patch 2 makes the material change of allowing a guest to actually toggle the OSLK bit by redirecting writes to OSLAR_EL1.OSLK to OSLSR_EL1.OSLK. When saving context, simply stash the value of OSLSR_EL1. On resume, apply OSLSR_EL1.OSLK to OSLAR_EL1.OSLK. Finally, Patch 3 raises the KVM debug architecture to v8.2 and exposes FEAT_DoubleLock as NI to the guest. With the changes to OSLAR_EL1 in this series, KVM now does what it says on the tin. This series applies cleanly to 5.15-rc4, and was (lightly) tested by booting 5.15-rc4 as a kvmtool guest on this kernel. Oliver Upton (3): KVM: arm64: Stash OSLSR_EL1 in the cpu context KVM: arm64: Allow the guest to change the OS Lock status KVM: arm64: Raise KVM's reported debug architecture to v8.2 arch/arm64/include/asm/kvm_host.h | 1 + arch/arm64/kvm/hyp/include/hyp/sysreg-sr.h | 5 +++ arch/arm64/kvm/sys_regs.c | 42 ++++++++++++++++------ 3 files changed, 37 insertions(+), 11 deletions(-) -- 2.33.0.1079.g6e70778dc9-goog
