On Thu, Aug 15, 2019 at 06:21:00PM +0000, Koenig, Christian wrote: > > (2) Add support for DMA_ATTR_NO_KERNEL_MAPPING to this new API instead > > of dma_alloc_attrs. The initial difference with that flag is just > > that we allow highmem, but in the future we could also unmap this > > memory from the kernel linear mapping entirely on architectures > > where we can easily do that. > > Mhm, why would we want to do this?
To avoid the CPU misspeculating into this memory. For example NVMe SSDs have a feature called host memory buffer that is a lot like your stolen main ram for the GPU case. We currently hand the SSD a DMA_ATTR_NO_KERNEL_MAPPING allocation if it requests such a buffer. If possible we'd really like to make sure no speculative execution bug (or intentional attacker with a kernel exploit for that matter) can easily access that memory.
