Robin Murphy
2021-Aug-03 10:53 UTC
[PATCH v10 01/17] iova: Export alloc_iova_fast() and free_iova_fast()
On 2021-08-03 09:54, Yongji Xie wrote:> On Tue, Aug 3, 2021 at 3:41 PM Jason Wang <jasowang at redhat.com> wrote: >> >> >> ? 2021/7/29 ??3:34, Xie Yongji ??: >>> Export alloc_iova_fast() and free_iova_fast() so that >>> some modules can use it to improve iova allocation efficiency. >> >> >> It's better to explain why alloc_iova() is not sufficient here. >> > > Fine.What I fail to understand from the later patches is what the IOVA domain actually represents. If the "device" is a userspace process then logically the "IOVA" would be the userspace address, so presumably somewhere you're having to translate between this arbitrary address space and actual usable addresses - if you're worried about efficiency surely it would be even better to not do that? Presumably userspace doesn't have any concern about alignment and the things we have to worry about for the DMA API in general, so it's pretty much just allocating slots in a buffer, and there are far more effective ways to do that than a full-blown address space manager. If you're going to reuse any infrastructure I'd have expected it to be SWIOTLB rather than the IOVA allocator. Because, y'know, you're *literally implementing a software I/O TLB* ;) Robin.