On Mon, Nov 07, 2022 at 01:03:31PM +0000, Robin Murphy wrote: > On 2022-11-06 22:01, Catalin Marinas wrote: >> With all the infrastructure in place for bouncing small kmalloc() >> buffers, add the corresponding Kconfig entry and select it for arm64. > > AFAICS we're missing the crucial part to ensure that SWIOTLB is available > even when max_pfn <= arm64_dma_phys_limit, which is very likely to be true > on low-memory systems that care most about kmalloc wastage. The only way to > override that currently is with "swiotlb=force", but bouncing *everything* > is not desirable either. FYI, one of the reasons for the swiotlb_init refactor that passes flags and a boolean a while ago is that we can trivially just either pass another flag or check a condition in swiotlb_init to allocate the buffer. There's actually another case for which we need the unconditional allocation, and that is the bouncing for untrusted external devices with dma-iommu.
