Re: [Xen-devel] [PATCH] Revert "swiotlb: remove SWIOTLB_MAP_ERROR"

[Christoph Hellwig]

On Fri, Mar 08, 2019 at 05:25:57PM +0000, Julien Grall wrote:
> In the common case, Dom0 also contains the PV backend drivers. Those 
> drivers may directly use the guest buffer in the DMA request (so a copy is 
> avoided). To avoid using a bounce buffer too much, xen-swiotlb will find 
> the host physical address associated to the guest buffer and will use it to 
> compute the DMA address.
>
> While Dom0 kernel may only deal with 32-bit physical address, the 
> hypervisor can still deal with up to 40-bit physical address. This means 
> the guest memory can be allocated above the 4GB threshold. Hence why the 
> dma_addr_t is always 64-bit with CONFIG_XEN=y.

This at least makes some sense.  But is it really so much better to
avoid having a 64-bit phys_addr_t?