Re: [PATCH v3 1/2] dma-direct: provide the ability to reserve per-numa CMA

[Christoph Hellwig]

On Wed, Jul 22, 2020 at 09:26:03PM +0000, Song Bao Hua (Barry Song) wrote:
> I understand your concern. Anyway, The primary purpose of this patchset is providing
> a general way for users like IOMMU to get local coherent dma buffers to put their
> command queue and page tables in. The first user case is what really made me
> begin to prepare this patchset.
> 
> For the second case, it is probably a positive side effect of this patchset for those users
> who have more concern on performance than dma security, then they maybe skip
> IOMMU by
> 	iommu.passthrough=
> 			[ARM64, X86] Configure DMA to bypass the IOMMU by default.
> 			Format: { "0" | "1" }
> 			0 - Use IOMMU translation for DMA.
> 			1 - Bypass the IOMMU for DMA.
> 			unset - Use value of CONFIG_IOMMU_DEFAULT_PASSTHROUGH.
> In this case, they can get local memory and get better performance.
> However, it is not the primary purpose of this patchset.

That's not what I mean.  Hardcoding the CMA regions in the kernel
config is just a bad idea, and we should not add more hard coded values.
You can always use CONFIG_CMDLINE to force a specific kernel command
line including your options.