[PATCH 0/2] KVM: arm64: Fix host-directed debug for non-protected pKVM guests

[Fuad Tabba <fuad.tabba@linux.dev>]

Hi folks,

Under pKVM, host-directed hardware debug of a non-protected guest
(KVM_GUESTDBG_USE_HW) and userspace single-step never fire.

When the host owns the debug registers, the world switch loads
MDSCR_EL1 from external_mdscr_el1, where the host's MDE/KDE/SS bits
live. flush_debug_state() copies the guest's debug_owner and the
owner-selected debug register state to the hyp vCPU, but not
external_mdscr_el1, so the field reaches hardware as zero and the debug
exceptions stay disabled. Patch 1 propagates it.

Patch 2 adds a debug-exceptions selftest for a userspace watchpoint,
which the suite did not cover: it checks that the watchpoint fires and
reports the accessed address in debug.arch.far. It fails without patch
1 and passes with it. The existing single-step test passes even when
stepping never fires, so it does not catch this.

Found while adding self-hosted debug support for protected VMs.

Based on Linux 7.2-rc4 (1590cf0329716).

Cheers,
/fuad

Fuad Tabba (2):
  KVM: arm64: Flush external_mdscr_el1 to the pKVM hyp vCPU
  KVM: arm64: selftests: Add a userspace watchpoint test

 arch/arm64/kvm/hyp/nvhe/hyp-main.c            | 10 ++++-
 .../selftests/kvm/arm64/debug-exceptions.c    | 41 +++++++++++++++++++
 2 files changed, 49 insertions(+), 2 deletions(-)


base-commit: 1590cf0329716306e948a8fc29f1d3ee87d3989f
-- 
2.39.5