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

Xuan Zhuo
2024-03-04T06:21:39Z

On Fri, 1 Mar 2024 08:41:50 -0500, "Michael S. Tsirkin" <mst@redhat.com> 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?

I checked some drivers(gve, mlx, ice), no one has the same operation like
virtio-net. We can do it because we have indirect feature.

Thanks.

>
> > 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
>