Messages in this thread | | | Date | Wed, 13 Mar 2019 19:32:54 +0100 | From | Christoph Hellwig <> | Subject | Re: [Xen-devel] [PATCH] Revert "swiotlb: remove SWIOTLB_MAP_ERROR" |
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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?
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