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

[Thierry Reding <thierry.reding@kernel.org>]

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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().

Thierry

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