Messages in this thread | | | Subject | Re: [PATCH v5 02/10] iommu/vt-d: Use per-device dma_ops | From | Lu Baolu <> | Date | Fri, 26 Jul 2019 09:56:51 +0800 |
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Hi,
On 7/25/19 7:43 PM, Christoph Hellwig wrote: > On Thu, Jul 25, 2019 at 03:18:03PM +0800, Lu Baolu wrote: >>> Don't we need to keep this bit so that we still allow the IOMMU >>> to act if the device has a too small DMA mask to address all memory in >>> the system, even if if it should otherwise be identity mapped? >>> >> >> This checking happens only when device is using an identity mapped >> domain. If the device has a small DMA mask, swiotlb will be used for >> high memory access. >> >> This is supposed to be handled in dma_direct_map_page(): >> >> if (unlikely(!dma_direct_possible(dev, dma_addr, size)) && >> !swiotlb_map(dev, &phys, &dma_addr, size, dir, attrs)) { >> report_addr(dev, dma_addr, size); >> return DMA_MAPPING_ERROR; >> } > > Well, yes. But the point is that the current code uses dynamic iommu > mappings even if the devices is in the identity mapped domain when the > dma mask іs too small to map all memory directly. Your change means it > will now use swiotlb which is most likely going to be a lot more
By default, we use DMA domain. The privileged users are able to change this with global kernel parameter or per-group default domain type under discussion. In another word, use of identity domain is a choice of the privileged user who should consider the possible bounce buffer overhead.
I think current code doesn't do the right thing. The user asks the iommu driver to use identity domain for a device, but the driver force it back to DMA domain because of the device address capability.
> expensive. I don't think that this change is a good idea, and even if > we decide that this is a good idea after all that should be done in a > separate prep patch that explains the rationale.
Yes. Make sense.
Best regards, Baolu
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