arm64/boot: Disable trapping of PMZR_EL0 writes to EL2grafted

__init_el2_fgt2() writes one mask to both HDFGRTR2_EL2 and HDFGWTR2_EL2.
PMZR_EL0 is write-only, so its trap bit, nPMZR_EL0, exists only in
HDFGWTR2_EL2 and is therefore never set: a PMZR_EL0 write from the host
traps to EL2, where the nVHE hypervisor has no handler and BUG()s. The
kernel never writes PMZR_EL0, but kernel.perf_user_access=1 has the PMU
driver set PMUSERENR_EL0.UEN for a task with a user-read event, so a
write from EL0 reaches the trap and takes the host down without a panic
message.

Accumulate the HDFGWTR2_EL2 bits separately, as __init_el2_fgt() already
does for HDFGWTR_EL2, and set nPMZR_EL0 with the other FEAT_PMUv3p9
bits.

Fixes: 858c7bfcb35e1 ("arm64/boot: Enable EL2 requirements for FEAT_PMUv3p9")
Cc: stable@vger.kernel.org
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Reviewed-by: Oliver Upton <oupton@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>

1 files changed, 49 insertions, 0 deletions

diff --git a/Documentation/driver-api/firmware/firmware-usage-guidelines.rst b/Documentation/driver-api/firmware/firmware-usage-guidelines.rst
new file mode 100644
index 000000000..336e91228
--- /dev/null
+++ b/Documentation/driver-api/firmware/firmware-usage-guidelines.rst
@@ -0,0 +1,49 @@
+===================
+Firmware Guidelines
+===================
+
+Users switching to a newer kernel should *not* have to install newer
+firmware files to keep their hardware working. At the same time updated
+firmware files must not cause any regressions for users of older kernel
+releases.
+
+Drivers that use firmware from linux-firmware should follow the rules in
+this guide. (Where there is limited control of the firmware,
+i.e. company doesn't support Linux, firmwares sourced from misc places,
+then of course these rules will not apply strictly.)
+
+* Firmware files shall be designed in a way that it allows checking for
+  firmware ABI version changes. It is recommended that firmware files be
+  versioned with at least a major/minor version. It is suggested that
+  the firmware files in linux-firmware be named with some device
+  specific name, and just the major version. The firmware version should
+  be stored in the firmware header, or as an exception, as part of the
+  firmware file name, in order to let the driver detact any non-ABI
+  fixes/changes. The firmware files in linux-firmware should be
+  overwritten with the newest compatible major version. Newer major
+  version firmware shall remain compatible with all kernels that load
+  that major number.
+
+* If the kernel support for the hardware is normally inactive, or the
+  hardware isn't available for public consumption, this can
+  be ignored, until the first kernel release that enables that hardware.
+  This means no major version bumps without the kernel retaining
+  backwards compatibility for the older major versions.  Minor version
+  bumps should not introduce new features that newer kernels depend on
+  non-optionally.
+
+* If a security fix needs lockstep firmware and kernel fixes in order to
+  be successful, then all supported major versions in the linux-firmware
+  repo that are required by currently supported stable/LTS kernels,
+  should be updated with the security fix. The kernel patches should
+  detect if the firmware is new enough to declare if the security issue
+  is fixed.  All communications around security fixes should point at
+  both the firmware and kernel fixes. If a security fix requires
+  deprecating old major versions, then this should only be done as a
+  last option, and be stated clearly in all communications.
+
+* Firmware files that affect the User API (UAPI) shall not introduce
+  changes that break existing userspace programs. Updates to such firmware
+  must ensure backward compatibility with existing userspace applications.
+  This includes maintaining consistent interfaces and behaviors that
+  userspace programs rely on.