Messages in this thread | | | Date | Mon, 14 Nov 2022 17:38:00 +0000 | From | Catalin Marinas <> | Subject | Re: [PATCH] Revert "arm64: dma: Drop cache invalidation from arch_dma_prep_coherent()" |
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On Mon, Nov 14, 2022 at 03:14:21PM +0000, Robin Murphy wrote: > On 2022-11-14 14:11, Will Deacon wrote: > > On Mon, Nov 14, 2022 at 04:33:29PM +0530, Manivannan Sadhasivam wrote: > > > 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].
So I guess a DMA pool that's not mapped in the linear map, together with memremap() instead of vmap(), would work around the issue. But the driver needs fixing, not the arch code.
-- Catalin
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