[PATCH v3 0/2] KVM: arm64: Fix and test MMIO sign-extending loads

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

Hi folks,

Changes since v2 [1]:
  - Patch 2: dropped the second make_el0_accessible() call and the
    el0_be_loads_end symbol; the EL0 code page does not need its
    user-access bit set because it is fetched, not loaded. (sashiko)

Oliver's Reviewed-by is on patch 1 only: the code there is unchanged
since v1.

A sign-extending load (LDRSB/LDRSH/LDRSW) from emulated MMIO returns a
zero-extended value rather than the sign-extended one the architecture
requires; vcpu_data_host_to_guest() strips the sign bits when it masks
the data to the access width.

If my git archeology is right, the masking dates to 2014 (b30070862edbd,
big-endian support) and has been wrong ever since, but sign-extending
loads from device memory are rare enough that nobody hit it. Patch 1
fixes it; patch 2 adds a selftest so it doesn't regress.

Both passes run under QEMU, which reports mixed-endian support.

Based on Linux 7.2-rc2 (8cdeaa50eae8d).

Cheers,
/fuad

[1] https://lore.kernel.org/all/20260625144807.2603272-1-fuad.tabba@linux.dev/

Fuad Tabba (2):
  KVM: arm64: Fix sign-extension of MMIO loads
  KVM: arm64: selftests: Add MMIO sign-extending load test

 arch/arm64/kvm/mmio.c                         |   7 +-
 tools/testing/selftests/kvm/Makefile.kvm      |   1 +
 .../selftests/kvm/arm64/mmio_sign_ext.c       | 255 ++++++++++++++++++
 3 files changed, 260 insertions(+), 3 deletions(-)
 create mode 100644 tools/testing/selftests/kvm/arm64/mmio_sign_ext.c

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2.39.5