Messages in this thread | | | Subject | Re: [RFC PATCH v2 00/11] Add support to dma_map_sg for P2PDMA | From | Robin Murphy <> | Date | Fri, 12 Mar 2021 17:46:31 +0000 |
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On 2021-03-12 16:18, Logan Gunthorpe wrote: > > > On 2021-03-12 8:51 a.m., Robin Murphy wrote: >> On 2021-03-11 23:31, Logan Gunthorpe wrote: >>> Hi, >>> >>> This is a rework of the first half of my RFC for doing P2PDMA in >>> userspace >>> with O_DIRECT[1]. >>> >>> The largest issue with that series was the gross way of flagging P2PDMA >>> SGL segments. This RFC proposes a different approach, (suggested by >>> Dan Williams[2]) which uses the third bit in the page_link field of the >>> SGL. >>> >>> This approach is a lot less hacky but comes at the cost of adding a >>> CONFIG_64BIT dependency to CONFIG_PCI_P2PDMA and using up the last >>> scarce bit in the page_link. For our purposes, a 64BIT restriction is >>> acceptable but it's not clear if this is ok for all usecases hoping >>> to make use of P2PDMA. >>> >>> Matthew Wilcox has already suggested (off-list) that this is the wrong >>> approach, preferring a new dma mapping operation and an SGL >>> replacement. I >>> don't disagree that something along those lines would be a better long >>> term solution, but it involves overcoming a lot of challenges to get >>> there. Creating a new mapping operation still means adding support to >>> more >>> than 25 dma_map_ops implementations (many of which are on obscure >>> architectures) or creating a redundant path to fallback with dma_map_sg() >>> for every driver that uses the new operation. This RFC is an approach >>> that doesn't require overcoming these blocks. >> >> I don't really follow that argument - you're only adding support to two >> implementations with the awkward flag, so why would using a dedicated >> operation instead be any different? Whatever callers need to do if >> dma_pci_p2pdma_supported() says no, they could equally do if >> dma_map_p2p_sg() (or whatever) returns -ENXIO, no? > > The thing is if the dma_map_sg doesn't support P2PDMA then P2PDMA > transactions cannot be done, but regular transactions can still go > through as they always did. > > But replacing dma_map_sg() with dma_map_new() affects all operations, > P2PDMA or otherwise. If dma_map_new() isn't supported it can't simply > not support P2PDMA; it has to maintain a fallback path to dma_map_sg().
But AFAICS the equivalent fallback path still has to exist either way. My impression so far is that callers would end up looking something like this:
if (dma_pci_p2pdma_supported()) { if (dma_map_sg(...) < 0) //do non-p2p fallback due to p2p failure } else { //do non-p2p fallback due to lack of support }
at which point, simply:
if (dma_map_sg_p2p(...) < 0) //do non-p2p fallback either way
seems entirely reasonable. What am I missing?
Let's not pretend that overloading an existing API means we can start feeding P2P pages into any old subsystem/driver without further changes - there already *are* at least some that retry ad infinitum if DMA mapping fails (the USB layer springs to mind...) and thus wouldn't handle the PCI_P2PDMA_MAP_NOT_SUPPORTED case acceptably.
> Given that the inputs and outputs for dma_map_new() will be completely > different data structures this will be quite a lot of similar paths > required in the driver. (ie mapping a bvec to the input struct and the > output struct to hardware requirements) If a bug crops up in the old > dma_map_sg(), developers might not notice it for some time seeing it > won't be used on the most popular architectures.
Huh? I'm specifically suggesting a new interface that takes the *same* data structure (at least to begin with), but just gives us more flexibility in terms of introducing p2p-aware behaviour somewhat more safely. Yes, we already know that we ultimately want something better than scatterlists for representing things like this and dma-buf imports, but that hardly has to happen overnight.
Robin.
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