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.
