Re: [PATCH] iommu/dma: Respect SWIOTLB force_bounce

[Robin Murphy <robin.murphy@arm.com>]

On 02/05/2024 6:07 am, Christoph Hellwig wrote:
> On Wed, May 01, 2024 at 08:13:18PM +0000, T.J. Mercier wrote:
>> iommu_dma_map_page and iommu_dma_map_sg conditionally use SWIOTLB, but
>> checking if force_bounce is set for the device is not part of that
>> condition. Check if devices have requested to force SWIOTLB use as part
>> of deciding to take the existing SWIOTLB paths.
> 
> This fails to explain why you'd want this somewhat surprising behavior,
> and why you consider it a bug fix.

Indeed, it's rather intentional that the "swiotlb=force" argument 
doesn't affect iommu-dma, since that's primarily for weeding out drivers 
making dodgy assumptions about DMA addresses, and iommu-dma is 
inherently even better at that already.

Beyond that I think this change also seems likely to interact badly with 
CC_ATTR_GUEST_MEM_ENCRYPT on x86, where we invoke the SWIOTLB_FORCE flag 
for dma-direct, but expect that an IOMMU can provide a decrypted view 
in-place, thus bouncing in that path would be unnecessarily detrimental.

Thanks,
Robin.