On Thu, Jul 09, 2026 at 09:58:01PM +0200, Thierry Reding wrote: > On Tue, Jul 07, 2026 at 12:27:13PM +0100, Will Deacon wrote: > > On Mon, Jul 06, 2026 at 03:49:24PM +0200, Thierry Reding wrote: > > > 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 <[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(). > > > > > > > > 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? > > > > You'll need to measure it, but we've seen reports of double-digit > > percentage regressions in performance and power. As I said, the problem > > is that you need to split the linear map to 4k page at runtime to unmap > > the dynamic carveout, but that isn't something that can be done on most > > CPUs. Therefore you end up having to use page-granular mappings for the > > entire thing, similarly to how 'rodata_full' drives can_set_direct_map() > > and the perf/power hit affects everything. > > The VPR has fairly large alignment restrictions (1 MiB) and we do unmap > in fairly large chunks (512 MiB currently, but we can change that if it > is helpful) because we really want to avoid resizing operations, so the > tradeoff is between frequency of resize vs. potential memory wasted. > > Does that change anything with regards to performance?
Turns out that the system we need this for is very likely going to end up using 4 KiB pages anyway because it doesn't have a whole lot of RAM (which is the whole reason we want the VPR to be resizable in the first place). So it sounds like set_direct_map_*() is a good way forward. Still, wouldn't it potentially be much faster to unmap entire 2 MiB blocks at a time if we know the driver guarantees the alignment? Maybe the changes to add num_pages as an argument to set_direct_map_*() that Mike mentioned would already be an improvement because it avoids those gratuitous calls to can_set_direct_map(). Thierry
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