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Tsirkin" To: Robin Murphy Cc: Xuan Zhuo , linux-kernel@vger.kernel.org, Joerg Roedel , Will Deacon , Christoph Hellwig , Marek Szyprowski , iommu@lists.linux.dev, Zelin Deng Subject: Re: [RFC] dma-mapping: introduce dma_can_skip_unmap() Message-ID: <20240301194817-mutt-send-email-mst@kernel.org> References: <20240301071918.64631-1-xuanzhuo@linux.alibaba.com> <64be2e23-c526-45d3-bb7b-29e31241bbef@arm.com> <20240301064632-mutt-send-email-mst@kernel.org> <20240301082703-mutt-send-email-mst@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 > > > >