Re: [RFC] dma-mapping: introduce dma_can_skip_unmap()

Michael S. Tsirkin
2024-03-01T13:41:58Z

On Fri, Mar 01, 2024 at 12:42:39PM +0000, Robin Murphy wrote:
> On 2024-03-01 11:50 am, Michael S. Tsirkin wrote:
> > On Fri, Mar 01, 2024 at 11:38:25AM +0000, Robin Murphy wrote:
> > > Not only is this idea not viable, the entire premise seems flawed - the
> > > reasons for virtio needing to use the DMA API at all are highly likely to be
> > > the same reasons for it needing to use the DMA API *properly* anyway.
> >
> > The idea has nothing to do with virtio per se
>
> Sure, I can see that, but if virtio is presented as the justification for
> doing this then it's the justification I'm going to look at first. And the
> fact is that it *does* seem to have particular significance, since having up
> to 19 DMA addresses involved in a single transfer is very much an outlier
> compared to typical hardware drivers.

That's a valid comment. Xuan Zhuo do other drivers do this too,
could you check pls?

> Furthermore the fact that DMA API
> support was retrofitted to the established virtio design means I would
> always expect it to run up against more challenges than a hardware driver
> designed around the expectation that DMA buffers have DMA addresses.

It seems virtio can't drive any DMA changes then it's forever tainted?
Seems unfair - we retrofitted it years ago, enough refactoring happened
since then.

> > - we are likely not the
> > only driver that wastes a lot of memory (hot in cache, too) keeping DMA
> > addresses around for the sole purpose of calling DMA unmap. On a bunch
> > of systems unmap is always a nop and we could save some memory if there
> > was a way to find out. What is proposed is an API extension allowing
> > that for anyone - not just virtio.
>
> And the point I'm making is that that "always" is a big assumption, and in
> fact for the situations where it is robustly true we already have the
> DEFINE_DMA_UNMAP_{ADDR,LEN} mechanism.
> I'd consider it rare for DMA
> addresses to be stored in isolation, as opposed to being part of some kind
> of buffer descriptor (or indeed struct scatterlist, for an obvious example)
> that a driver or subsystem still has to keep track of anyway, so in general
> I believe the scope for saving decidedly small amounts of memory at runtime
> is also considerably less than you might be imagining.
>
> Thanks,
> Robin.

Yes. DEFINE_DMA_UNMAP_ exits but that's only compile time.
And I think the fact we have that mechanism is a hint that
enough configurations could benefit from a runtime
mechanism, too.

E.g. since you mentioned scatterlist, it has a bunch of ifdefs
in place.

Of course
- finding more examples would be benefitial to help maintainers
do the cost/benefit analysis
- a robust implementation is needed

--
MST