Re: [PATCH v3 04/11] arm64/mm: Add set_memory_device() and set_memory_normal()

[Will Deacon <will@kernel.org>]

On Thu, Jul 02, 2026 at 06:41:23PM +0200, Thierry Reding wrote:
> On Thu, Jul 02, 2026 at 03:46:44PM +0200, Thierry Reding wrote:
> > On Thu, Jul 02, 2026 at 10:18:47AM +0100, Will Deacon wrote:
> > > On Wed, Jul 01, 2026 at 06:08:15PM +0200, Thierry Reding wrote:
> > > > From: Chun Ng <chunn@nvidia.com>
> > > > 
> > > > Add helpers to swap PROT_NORMAL and PROT_DEVICE_nGnRnE protection bits
> > > > on a kernel-linear-map range.
> > > 
> > > That sounds like a really terrible idea. Why is this necessary and how
> > > does it interact with things like load_unaligned_zeropad()?
> > 
> > This is necessary because once the memory controller has walled off the
> > new memory region the CPU must not access it under any circumstances or
> > it'll cause the CPU to lock up (I think technically it'll hit an SError
> > but in practice that just means it'll freeze, as far as I can tell).
> > 
> > Probably doesn't interact well at all with load_unaligned_zeropad().
> > 
> > > I think you should unmap the memory from the linear map and memremap()
> > > it instead.
> > 
> > Given that the memory can never be accessed by the CPU after the memory
> > controller locks it down, I don't think we'll even need memremap(). The
> > only thing we really need is the sg_table we hand out via the DMA BUFs
> > so that they can be used by device drivers to program their DMA engines
> > internally.
> > 
> > Looking through some of the architecture code around this, shouldn't we
> > simply be using set_memory_encrypted() and set_memory_decrypted() for
> > this? While they might've been created for slightly other use-cases,
> > they seem to be doing exactly what we want (i.e. remove the page range
> > from the linear mapping and flushing it, or restoring the valid bit and
> > standard permissions, respectively).
> 
> Ah... I guess we can't do it because we're not in a realm world and so
> the early checks in __set_memory_enc_dec() would return early and turn
> it into a no-op.
> 
> How about if I extract a common helper and provide set_memory_p() and
> set_memory_np() in terms of those. Those are available on x86 and
> PowerPC as well, so fairly standard. I suppose at that point we're
> closer to set_memory_valid().

Why not just call set_direct_map_invalid_noflush() +
flush_tlb_kernel_range() for each page? We already have APIs for this.

The big challenge I see with any linear map manipulation, however, is
that it will rely on can_set_direct_map() which likely means you need to
give up some performance and/or security to make this work. Does memory
become inaccesible dynamically at runtime? If not, the best bet would
be to describe it as a carveout in the DT and mark it as "no-map" so
we avoid mapping it in the first place.

Will