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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [RFC PATCH 2/2] xen/grant-table: Use unpopulated DMAable pages instead of real RAM ones
On 11.06.22 02:55, Stefano Stabellini wrote: Hello Stefano On Thu, 9 Jun 2022, Oleksandr wrote:You can check if the page is within the mhp_range range or part ofOn 04.06.22 00:19, Stefano Stabellini wrote: Hello Stefano Thank you for having a look and sorry for the late response.On Tue, 17 May 2022, Oleksandr Tyshchenko wrote: I got your idea and agree this can work technically. Or if we finally decide to use the second option (use "dma_pool" for all) in the first patch "[RFC PATCH 1/2] xen/unpopulated-alloc: Introduce helpers for DMA allocations" then we will likely be able to check whether a page in question is within a "dma_pool" using gen_pool_has_addr().I am still wondering, can we avoid the fallback implementation in unpopulated-alloc.c. Because for that purpose we will need to pull more code into unpopulated-alloc.c (to be more precise, almost everything which gnttab_dma_free_pages() already has except gnttab_pages_clear_private()) and pass more arguments to xen_free_unpopulated_dma_pages(). Also I might mistake, but having a fallback split between grant-table.c (to allocate RAM pages in gnttab_dma_alloc_pages()) and unpopulated-alloc.c (to free RAM pages in xen_free_unpopulated_dma_pages()) would look a bit weird. I see two possible options for the fallback implementation in grant-table.c: 1. (less preferable) by introducing new flag in struct gnttab_dma_alloc_args2. (more preferable) by guessing unpopulated (non real RAM) page using is_zone_device_page(), etc For example, with the second option the resulting code will look quite simple (only build tested): diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c index 738029d..3bda71f 100644 --- a/drivers/xen/grant-table.c +++ b/drivers/xen/grant-table.c@@ -1047,6 +1047,23 @@ int gnttab_dma_alloc_pages(struct gnttab_dma_alloc_args *args)
size_t size;
int i, ret;
+ if (IS_ENABLED(CONFIG_XEN_UNPOPULATED_ALLOC)) {
+ ret = xen_alloc_unpopulated_dma_pages(args->dev,
args->nr_pages,
+ args->pages); + if (ret < 0) + goto fallback; + + ret = gnttab_pages_set_private(args->nr_pages, args->pages); + if (ret < 0) + goto fail; + + args->vaddr = page_to_virt(args->pages[0]); + args->dev_bus_addr = page_to_phys(args->pages[0]); + + return ret; + } + +fallback: size = args->nr_pages << PAGE_SHIFT; if (args->coherent) args->vaddr = dma_alloc_coherent(args->dev, size,@@ -1103,6 +1120,12 @@ int gnttab_dma_free_pages(struct gnttab_dma_alloc_args *args)
gnttab_pages_clear_private(args->nr_pages, args->pages);
+ if (IS_ENABLED(CONFIG_XEN_UNPOPULATED_ALLOC) &&
+ is_zone_device_page(args->pages[0])) {
+ xen_free_unpopulated_dma_pages(args->dev,
args->nr_pages, args->pages);
+ return 0; + } + for (i = 0; i < args->nr_pages; i++) args->frames[i] = page_to_xen_pfn(args->pages[i]); What do you think?
ok, thank you for the confirmation. + return ret; + } + + args->vaddr = page_to_virt(args->pages[0]); + args->dev_bus_addr = page_to_phys(args->pages[0]);There are two things to note here. The first thing to note is that normally we would call pfn_to_bfn to retrieve the dev_bus_addr of a page because pfn_to_bfn takes into account foreign mappings. However, these are freshly allocated pages without foreign mappings, so page_to_phys/dma should be sufficient.agreeThe second has to do with physical addresses and DMA addresses. The functions are called gnttab_dma_alloc_pages and xen_alloc_unpopulated_dma_pages which make you think we are retrieving a DMA address here. However, to get a DMA address we need to call page_to_dma rather than page_to_phys. page_to_dma takes into account special offsets that some devices have when accessing memory. There are real cases on ARM where the physical address != DMA address, e.g. RPi4.I got it. Now I am in doubt whether it would be better to name the API: xen_alloc_unpopulated_cma_pages() or xen_alloc_unpopulated_contiguous_pages() What do you think?Yeah actually I think it is better to stay away from "dma" in the name. I like xen_alloc_unpopulated_contiguous_pages(). perfect, I will rename then, thank you for the confirmation. However, to call page_to_dma you need to specify as first argument the DMA-capable device that is expected to use those pages for DMA (e.g. an ethernet device or a MMC controller.) While the args->dev we have in gnttab_dma_alloc_pages is the gntdev_miscdev.agree As I understand, at this time it is unknown for what exactly device these pages are supposed to be used at the end. For now, it is only known that these pages to be used by userspace PV backend for grant mappings.YeahIn that case, an IOMMU is required. If an IOMMU is setup correct, thenSo this interface cannot actually be used to allocate memory that is supposed to be DMA-able by a DMA-capable device, such as an ethernet device.agreeBut I think that should be fine because the memory is meant to be used by a userspace PV backend for grant mappings. If any of those mappings end up being used for actual DMA in the kernel they should go through the drivers/xen/swiotlb-xen.c and xen_phys_to_dma should be called, which ends up calling page_to_dma as appropriate. It would be good to double-check that the above is correct and, if so, maybe add a short in-code comment about it: /* * These are not actually DMA addresses but regular physical addresses. * If these pages end up being used in a DMA operation then the * swiotlb-xen functions are called and xen_phys_to_dma takes care of * the address translations: * * - from gfn to bfn in case of foreign mappings * - from physical to DMA addresses in case the two are different for a * given DMA-mastering device */I agree this needs to be re-checked. But, there is one moment here, if userspace PV backend runs in other than Dom0 domain (non 1:1 mapped domain), the xen-swiotlb seems not to be in use then? How to be in this case?the gfn->bfn translation is not necessary because it is done automatically by the IOMMU. That is because when the foreign page is mapped in the domain, the mapping also applies to the IOMMU pagetable. So the device is going to do DMA to "gfn" and the IOMMU will translate it to the right "mfn", the one corresponding to "bfn". The physical to DMA address should be done automatically by the default (non-swiotlb_xen) dma_ops in Linux. E.g. kernel/dma/direct.c:dma_direct_map_sg correctly calls dma_direct_map_page, which calls phys_to_dma. Thank you for the explanation.
-- Regards, Oleksandr Tyshchenko
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