On Tue, 06 May 2025 15:25:09 +0100, Will Deacon <will@xxxxxxxxxx> wrote: > > On Mon, Apr 28, 2025 at 03:35:14PM +0000, Mikołaj Lenczewski wrote: > > The Break-Before-Make cpu feature supports multiple levels (levels 0-2), > > and this commit adds a dedicated BBML2 cpufeature to test against > > support for, as well as a kernel commandline parameter to optionally > > disable BBML2 altogether. > > > > This is a system feature as we might have a big.LITTLE architecture > > where some cores support BBML2 and some don't, but we want all cores to > > be available and BBM to default to level 0 (as opposed to having cores > > without BBML2 not coming online). > > > > To support BBML2 in as wide a range of contexts as we can, we want not > > only the architectural guarantees that BBML2 makes, but additionally > > want BBML2 to not create TLB conflict aborts. Not causing aborts avoids > > us having to prove that no recursive faults can be induced in any path > > that uses BBML2, allowing its use for arbitrary kernel mappings. > > Support detection of such CPUs. > > > > Signed-off-by: Mikołaj Lenczewski <miko.lenczewski@xxxxxxx> > > Reviewed-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx> > > Reviewed-by: Ryan Roberts <ryan.roberts@xxxxxxx> > > --- > > .../admin-guide/kernel-parameters.txt | 3 + > > arch/arm64/Kconfig | 19 +++++ > > arch/arm64/include/asm/cpucaps.h | 2 + > > arch/arm64/include/asm/cpufeature.h | 5 ++ > > arch/arm64/kernel/cpufeature.c | 71 +++++++++++++++++++ > > arch/arm64/kernel/pi/idreg-override.c | 2 + > > arch/arm64/tools/cpucaps | 1 + > > 7 files changed, 103 insertions(+) > > > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt > > index d9fd26b95b34..2749c67a4f07 100644 > > --- a/Documentation/admin-guide/kernel-parameters.txt > > +++ b/Documentation/admin-guide/kernel-parameters.txt > > @@ -449,6 +449,9 @@ > > arm64.no32bit_el0 [ARM64] Unconditionally disable the execution of > > 32 bit applications. > > > > + arm64.nobbml2 [ARM64] Unconditionally disable Break-Before-Make Level > > + 2 support > > Hmm, I'm not sure we really want this. It opens up the door for folks to > pass 'id_aa64mmfr2.bbm=2' without updating the allow-list which feels > like it's going to make crashes harder to reason about. Passing id_aa64mmfr2.bbm=2 shouldn't have any effect if the HW doesn't advertise it already, as you can only downgrade features. Trying to upgrade features should leave a nastygram in the kernel log. M. -- Without deviation from the norm, progress is not possible.
