Re: [PATCH v8 0/5] arm64: support FEAT_BBM level 2 and large block mapping when rodata=full

[Will Deacon <will@kernel.org>]

On Fri, Sep 19, 2025 at 11:08:47AM +0100, Ryan Roberts wrote:
> On 18/09/2025 22:10, Will Deacon wrote:
> > On Wed, 17 Sep 2025 12:02:06 -0700, Yang Shi wrote:
> >> On systems with BBML2_NOABORT support, it causes the linear map to be mapped
> >> with large blocks, even when rodata=full, and leads to some nice performance
> >> improvements.
> >>
> >> Ryan tested v7 on an AmpereOne system (a VM with 12G RAM) in all 3 possible
> >> modes by hacking the BBML2 feature detection code:
> >>
> >> [...]
> > 
> > Applied patches 1 and 3 to arm64 (for-next/mm), thanks!
> > 
> > [1/5] arm64: Enable permission change on arm64 kernel block mappings
> >       https://git.kernel.org/arm64/c/a660194dd101
> > [3/5] arm64: mm: support large block mapping when rodata=full
> >       https://git.kernel.org/arm64/c/a166563e7ec3
> > 
> > I also picked up the BBML allow-list addition (second patch) on
> > for-next/cpufeature.
> > 
> > The fourth patch ("arm64: mm: split linear mapping if BBML2 unsupported
> > on secondary CPUs") has some really horrible conflicts. These are partly
> > due to some of the type cleanups on for-next/mm but I think mainly due
> > to Kevin's kpti rework that landed after -rc1.
> 
> Thanks Will, although I'm nervous that without this patch, some platforms might
> not boot; Wikipedia tells me that there are some Google, Mediatek and Qualcomm
> SoCs that pair X4 CPUs (which is on the BBML2_NOABORT allow list) with A720
> and/or A520 (which are not). See previous mail at [1].

I'd be surprised if these SoCs are booting on the X4 but who knows.

Lemme have another look at applying the patch with fresh eyes, but I do
wonder whether having X4 on the allow list really makes any sense. Are
there any SoCs out there that _don't_ pair it with CPUs that aren't on
the allow list? (apologies for the double negative).

Will