Re: [PATCH v1 1/3] arm64: Add BBM Level 2 cpu feature

[Robin Murphy]

On 2025-02-19 3:43 pm, Ryan Roberts wrote:
> On 19/02/2025 15:39, Robin Murphy wrote:
> > Hi Miko,
> >
> > On 2025-02-19 2:38 pm, 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.
> > >
> > > 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.
> >
> > If this may be used for splitting/compacting userspace mappings, then similarly
> > to 6e192214c6c8 ("iommu/arm-smmu-v3: Document SVA interaction with new pagetable
> > features"), strictly we'll also want a check in arm_smmu_sva_supported() to make
> > sure that the SMMU is OK with BBML2 behaviour too, and disallow SVA if not. Note
> > that the corresponding SMMUv3.2-BBML2 feature is already strict about TLB
> > conflict aborts, so is comparatively nice and straightforward.
>
> Thanks for catching this, Robin, as I completely forgot to pass this onto Miko
> yesterday after our conversation. I suggest we tack a commit on to the end of
> this series to cover that?
>
> I think that strictly this is not needed for Yang's series since that only uses
> BBML2 for kernel mappings, and those pgtables would never be directly shared
> with the SMMU.

Yup, it's really more just a theoretical correctness concern - certainly 
Arm's implementations from MMU-700 onwards do support BBML2, while 
MMU-600 is now sufficiently old that nobody is likely to pair it with 
new BBML-capable CPUs anyway - so it's just to cover the gap that in 
principle there may be 3rd-party implementations which might get confused.

Cheers,
Robin.