Re: [PATCH v5 02/10] iommu/vt-d: Use per-device dma_ops

[Lu Baolu]

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 Ñs 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