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

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Mar 01, 2024 at 06:04:10PM +0000, Robin Murphy wrote:
> On 2024-03-01 1:41 pm, Michael S. Tsirkin wrote:
> > 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.
> 
> No, I'm not saying we couldn't still do things to help virtio if and when it
> does prove reasonable to do so; just that if anything it's *because* that
> retrofit is mature and fairly well polished by now that any remaining issues
> like this one are going to be found in the most awkward corners and thus
> unlikely to generalise.
> 
> FWIW in my experience it seems more common for network drivers to actually
> have the opposite problem, where knowing the DMA address of a buffer is
> easy, but keeping track of the corresponding CPU address can be more of a
> pain.
> 
> > > > - 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.
> 
> But what could that benefit be in general? It's not like we can change
> structure layouts on a per-DMA-mapping-call basis to save already-allocated
> memory... :/
> 
> Thanks,
> Robin.

This is all speculation, but maybe e.g. by not writing into a cache line
we can reduce pressure on the cache.  Some other code and/or structure
changes might or might not become benefitial.


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