On Tue, 28 Mar 2023 09:54:35 +0200 Petr Tesarik <petrtesarik@xxxxxxxxxxxxxxx> wrote: > On 3/28/2023 6:07 AM, Christoph Hellwig wrote: > > [adding Alex as he has been interested in this in the past] > > > > On Mon, Mar 20, 2023 at 01:28:15PM +0100, Petr Tesarik wrote: > >> Second, on the Raspberry Pi 4, swiotlb is used by dma-buf for pages > >> moved from the rendering GPU (v3d driver), which can access all > >> memory, to the display output (vc4 driver), which is connected to a > >> bus with an address limit of 1 GiB and no IOMMU. These buffers can > >> be large (several megabytes) and cannot be handled by SWIOTLB, > >> because they exceed maximum segment size of 256 KiB. Such mapping > >> failures can be easily reproduced on a Raspberry Pi4: Starting > >> GNOME remote desktop results in a flood of kernel messages like > >> these: > > > > Shouldn't we make sure dma-buf allocates the buffers for the most > > restricted devices, and more importantly does something like a dma > > coherent allocation instead of a dynamic mapping of random memory? > > > > While a larger swiotlb works around this I don't think this fixes the root > > cause. > > I tend to agree here. However, it's the DMABUF design itself that causes > some trouble. The buffer is allocated by the v3d driver, which does not > have the restriction, so the DMA API typically allocates an address > somewhere near the 4G boundary. Userspace then exports the buffer, sends > it to another process as a file descriptor and imports it into the vc4 > driver, which requires DMA below 1G. In the beginning, v3d had no idea > that the buffer would be exported to userspace, much less that it would > be later imported into vc4. > > Anyway, I suspected that the buffers need not be imported into the vc4 > driver (also hinted by Eric Anholt in a 2018 blog post [1]), and it > seems I was right. I encountered the issue with Ubuntu 22.10; I > installed latest openSUSE Tumbleweed yesterday, and I was not able to > reproduce the issue there, most likely because the Mesa drivers have > been fixed meanwhile. This makes the specific case of the Raspberry Pi 4 > drivers moot. The issue may still affect other combinations of drivers, > but I don't have any other real-world example ATM. I'm only seeing this problem with Wayland, no issue when switching Ubuntu to X. It seems Tumbleweed is using X by default. ...Juerg > [1] https://anholt.github.io/twivc4/2018/02/12/twiv/ > > >> 1. The value is limited to ULONG_MAX, which is too little both for > >> physical addresses (e.g. x86 PAE or 32-bit ARM LPAE) and DMA > >> addresses (e.g. Xen guests on 32-bit ARM). > >> > >> 2. Since buffers are currently allocated with page granularity, a > >> PFN can be used instead. However, some values are reserved by > >> the maple tree implementation. Liam suggests to use > >> xa_mk_value() in that case, but that reduces the usable range by > >> half. Luckily, 31 bits are still enough to hold a PFN on all > >> 32-bit platforms. > >> > >> 3. Software IO TLB is used from interrupt context. The maple tree > >> implementation is not IRQ-safe (MT_FLAGS_LOCK_IRQ does nothing > >> AFAICS). Instead, I use an external lock, spin_lock_irqsave() and > >> spin_unlock_irqrestore(). > >> > >> Note that bounce buffers are never allocated dynamically if the > >> software IO TLB is in fact a DMA restricted pool, which is intended > >> to be stay in its designated location in physical memory. > > > > I'm a little worried about all that because it causes quite a bit > > of overhead even for callers that don't end up going into the > > dynamic range or do not use swiotlb at all. I don't really have a > > good answer here except for the usual avoid bounce buffering whenever > > you can that might not always be easy to do. > > I'm also worried about all this overhead. OTOH I was not able to confirm > it, because the difference between two successive fio test runs on an > unmodified kernel was bigger than the difference between a vanilla and a > patched kernel, except the maximum completion latency, which OTOH > affected less than 0.01% of all requests. > > BTW my testing also suggests that the streaming DMA API is quite > inefficient, because UAS performance _improved_ with swiotlb=force. > Sure, this should probably be addressed in the UAS and/or xHCI driver, > but what I mean is that moving away from swiotlb may even cause > performance regressions, which is counter-intuitive. At least I would > _not_ have expected it. > > >> + gfp = (attrs & DMA_ATTR_MAY_SLEEP) ? GFP_KERNEL : GFP_NOWAIT; > >> + slot = kmalloc(sizeof(*slot), gfp | __GFP_NOWARN); > >> + if (!slot) > >> + goto err; > >> + > >> + slot->orig_addr = orig_addr; > >> + slot->alloc_size = alloc_size; > >> + slot->page = dma_direct_alloc_pages(dev, PAGE_ALIGN(alloc_size), > >> + &slot->dma_addr, dir, > >> + gfp | __GFP_NOWARN); > >> + if (!slot->page) > >> + goto err_free_slot; > > > > Without GFP_NOIO allocations this will deadlock eventually. > > Ah, that would affect the non-sleeping case (GFP_KERNEL), right? > > Petr T >
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