[PATCH 0/2] KVM: arm64: Stop advertising FEAT_LS64_ACCDATA to guests

[Fuad Tabba]

Hi folks,

On a host with FEAT_LS64_ACCDATA, KVM advertises ST64BV0 and ACCDATA_EL1
to guests but supports neither. The first patch caps
ID_AA64ISAR1_EL1.LS64 at LS64_V. A VM saved on an older kernel can carry
any level up to the host's, so the patch still accepts those levels from
the VMM, capped at LS64_V, and the VM restores. The second patch moves
LS64 out of set_id_regs' generic table into a test of its own, since the
table's invalid value is now one of those levels. It carries Cc: stable
as well, because a stable tree that takes the first patch alone fails
the set_id_regs selftest on an LS64_ACCDATA host.

Tested on the FVP AEM with arm_v8_7_accelerator_support_level set to 3
(LS64_ACCDATA) and 4 (LS64WB). Unpatched, on a VHE host, a guest's
ST64BV0 hits the WARN in handle_other(). A second VM also reads the
ACCDATA_EL1 value the first one wrote, once cpuidle.off=1 stops the
FVP's idle states from powering the CPU down. Patched, on VHE, nVHE and
protected-mode hosts, ST64BV0 and the ACCDATA_EL1 accesses UNDEF, and
set_id_regs passes.

The cap alone would fix the bug, but a VM saved on an older kernel
would then fail to restore on a patched kernel. 6685f5d572c2 ("KVM:
arm64: Disable MPAM visibility by default and ignore VMM writes")
solved the same problem for MPAM: it hid the field from guests, but
kept accepting the value older kernels exposed.

Based on Linux 7.3-rc3 (fd73f4a665989). Patch 2 conflicts with
00e309933879c, now in kvmarm/fixes and kvmarm/next, on the test count.
The resolution is:

  -	test_cnt = 4 + MPAM_IDREG_TEST + MTE_IDREG_TEST;
  +	test_cnt = 4 + MPAM_IDREG_TEST + MTE_IDREG_TEST + LS64_IDREG_TEST;

Cheers,
/fuad

Fuad Tabba (2):
  KVM: arm64: Hide FEAT_LS64_ACCDATA from guests
  KVM: arm64: selftests: Test ID_AA64ISAR1_EL1.LS64 on its own

 arch/arm64/kvm/sys_regs.c                     | 28 +++++++++--
 .../testing/selftests/kvm/arm64/set_id_regs.c | 49 ++++++++++++++++++-
 2 files changed, 71 insertions(+), 6 deletions(-)


base-commit: fd73f4a6659897191fa0d40695fe370925dd3780
-- 
2.47.3