On Fri, Jul 03, 2026 at 06:13:31PM +0100, Will Deacon wrote:
> 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.

Having a "standard" helper with a fixed and documented purposed seemed
like a preferable approach for this particular case. We also may want to
make the driver that uses this buildable as a module, in which case we'd
need to export these rather low-level APIs. And then there's also the
fact that we typically call this on a rather large region of memory
(usually something like 512 MiB), so doing it page-by-page is rather
suboptimal.

> 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.

VPR exists in two modes: static and resizable. For static VPR we do
exactly that: describe it as carveout in DT with no-map and deal with it
accordingly in the driver. Resizable VPR is for device that have small
amounts of RAM. Content-protected video playback will in the worst case
consume around 1.8 GiB of RAM, so we want to be able to reuse for other
purposes when VPR is unused on those devices. In that case, the memory
is also described as a reserved-memory region in DT, but it is marked as
reusable so that it can be managed by CMA.

The resize operation is fairly slow to begin with because we need to
stall the GPU and put it into reset before the operation, then take it
out of reset and resume it afterwards.

What kind of performance impact do you expect?

Thierry