Re: [PATCH] Revert "arm64: dma: Drop cache invalidation from arch_dma_prep_coherent()"
From: Robin Murphy <robin.murphy@arm.com>
Date: 2022-11-14 15:14:34
Also in:
lkml
On 2022-11-14 14:11, Will Deacon wrote:
On Mon, Nov 14, 2022 at 04:33:29PM +0530, Manivannan Sadhasivam wrote:quoted
This reverts commit c44094eee32f32f175aadc0efcac449d99b1bbf7. As reported by Amit [1], dropping cache invalidation from arch_dma_prep_coherent() triggers a crash on the Qualcomm SM8250 platform (most probably on other Qcom platforms too). The reason is, Qcom qcom_q6v5_mss driver copies the firmware metadata and shares it with modem for validation. The modem has a secure block (XPU) that will trigger a whole system crash if the shared memory is accessed by the CPU while modem is poking at it. To avoid this issue, the qcom_q6v5_mss driver allocates a chunk of memory with no kernel mapping, vmap's it, copies the firmware metadata and unvmap's it. Finally the address is then shared with modem for metadata validation [2]. Now because of the removal of cache invalidation from arch_dma_prep_coherent(), there will be cache lines associated with this memory even after sharing with modem. So when the CPU accesses it, the XPU violation gets triggered.This last past is a non-sequitur: the buffer is no longer mapped on the CPU side, so how would the CPU access it?
Right, for the previous change to have made a difference the offending part of this buffer must be present in some cache somewhere *before* the DMA buffer allocation completes. Clearly that driver is completely broken though. If the DMA allocation came from a no-map carveout vma_dma_alloc_from_dev_coherent() then the vmap() shenanigans wouldn't work, so if it backed by struct pages then the whole dance is still pointless because *a cacheable linear mapping exists*, and it's just relying on the reduced chance that anything's going to re-fetch the linear map address after those pages have been allocated, exactly as I called out previously[1]. Robin. [1] https://lore.kernel.org/linux-arm-kernel/97fface8-e40e-072c-4335-c94094884e93@arm.com/ (local)
As I just replied to Amit, we need more information about what this "access" is and how it is being detected. Will
_______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel