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[v2,0/4] target/arm: Make the counter frequency default 1GHz for new CPUs, machines

Message ID 20240426122913.3427983-1-peter.maydell@linaro.org
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Series target/arm: Make the counter frequency default 1GHz for new CPUs, machines | expand

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Peter Maydell April 26, 2024, 12:29 p.m. UTC 
In previous versions of the Arm architecture, the frequency of the
generic timers as reported in CNTFRQ_EL0 could be any IMPDEF value,
and for QEMU we picked 62.5MHz, giving a timer tick period of 16ns.
In Armv8.6, the architecture standardized this frequency to 1GHz.

Because there is no ID register feature field that indicates whether a
CPU is v8.6 or that it ought to have this counter frequency, we
implement this by changing our default CNTFRQ value for all CPUs, with
exceptions for backwards compatibility:

 * CPU types which we already implement will retain the old
   default value. None of these are v8.6 CPUs, so this is
   architecturally OK.
 * CPUs used in versioned machine types with a version of 9.0
   or earlier will retain the old default value.

The upshot is that the only CPU type that changes is 'max'; but any
new type we add in future (whether v8.6 or not) will also get the new
1GHz default (assuming we spot in code review any attempts to set
the ARM_FEATURE_BACKCOMPAT_CNTFRQ flag on new CPU types as a result
of cut-n-paste from an older CPU initfn ;-)).

It remains the case that the machine model can override the default
value via the 'cntfrq' QOM property (regardless of the CPU type).

Unfortunately the TF-A firmware used to hard-code the CPU frequency,
resulting in guest timers not running for the right duration. This
is fixed in TF-A git but not yet in a release, and affects users
running TF-A on either virt or sbsa-ref. For virt I think running
TF-A is not a common setup, and besides we have versioned board
models so users can use virt-9.0 if they want to run older TF-A
binaries. For sbsa-ref the machine isn't versioned and TF-A is
part of the standard guest software stack, so I've opted in this
patchset to have our board model retain the old 62.5MHz clock for
now. We can update that once e.g. TF-A has made a release with
the fix (and we've updated our Avocado test's binaries!). I
plan to leave it up to the sbsa-ref maintainers to decide when
they're happy to make that change.

Patches 1 and 4 are from v1 and have been reviewed.
Patches 2 and 3 are new and together keep sbsa-ref on the old
62.5MHz value, at least for now.

thanks
-- PMM


Peter Maydell (4):
  target/arm: Refactor default generic timer frequency handling
  hw/arm/sbsa-ref: Force CPU generic timer to 62.5MHz
  hw/watchdog/sbsa_gwdt: Make watchdog timer frequency a QOM property
  target/arm: Default to 1GHz cntfrq for 'max' and new CPUs

 include/hw/watchdog/sbsa_gwdt.h |  3 +--
 target/arm/cpu.h                | 11 +++++++++
 target/arm/internals.h          | 15 +++++++++---
 hw/arm/sbsa-ref.c               | 16 +++++++++++++
 hw/core/machine.c               |  4 +++-
 hw/watchdog/sbsa_gwdt.c         | 15 +++++++++++-
 target/arm/cpu.c                | 42 ++++++++++++++++++++++-----------
 target/arm/cpu64.c              |  2 ++
 target/arm/helper.c             | 16 ++++++-------
 target/arm/tcg/cpu32.c          |  4 ++++
 target/arm/tcg/cpu64.c          | 18 ++++++++++++++
 11 files changed, 117 insertions(+), 29 deletions(-)