Re: [RFC PATCH] arm64: remove CONFIG_DEBUG_ALIGN_RODATA feature

[Will Deacon <will@kernel.org>]

On Thu, Apr 02, 2020 at 12:30:33PM +0100, Catalin Marinas wrote:
> On Mon, Mar 30, 2020 at 04:32:31PM +0200, Ard Biesheuvel wrote:
> > On Mon, 30 Mar 2020 at 16:28, Will Deacon <will@kernel.org> wrote:
> > > > On Mon, 30 Mar 2020 at 16:04, Will Deacon <will@kernel.org> wrote:
> > > > > On Mon, Mar 30, 2020 at 03:53:04PM +0200, Ard Biesheuvel wrote:
> > > > > > On Mon, 30 Mar 2020 at 15:51, Will Deacon <will@kernel.org> wrote:
> > > > > > > But I would really like to go a step further and rip out the block mapping
> > > > > > > support altogether so that we can fix non-coherent DMA aliases:
> > > > > > >
> > > > > > > https://lore.kernel.org/lkml/20200224194446.690816-1-hch@lst.de
> > > > > >
> > > > > > I'm not sure I follow - is this about mapping parts of the static
> > > > > > kernel Image for non-coherent DMA?
> > > > >
> > > > > Sorry, it's not directly related to your patch, just that if we're removing
> > > > > options relating to kernel mappings then I'd be quite keen on effectively
> > > > > forcing page-granularity on the linear map, as is currently done by default
> > > > > thanks to RODATA_FULL_DEFAULT_ENABLED, so that we can nobble cacheable
> > > > > aliases for non-coherent streaming DMA mappings by hooking into Christoph's
> > > > > series above.
> 
> Have we ever hit this issue in practice? At least from the CPU
> perspective, we've assumed that a non-cacheable access would not hit in
> the cache. Reading the ARM ARM rules, it doesn't seem to state this
> explicitly but we can ask for clarification (I dug out an email from
> 2015, left unanswered).
> 
> Assuming that the CPU is behaving as we'd expect, are there other issues
> with peripherals/SMMU?

Any clarification would need to be architectural, I think, and not specific
to the CPU. Worth asking again though.

Will