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 <[email protected]> > > > > > > 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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