On Tue, 17 May 2022, Oleksandr Tyshchenko wrote:
From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>This is an alternative implementation of the same function.
Depends on CONFIG_XEN_UNPOPULATED_ALLOC. If enabled then unpopulated
DMAable (contiguous) pages will be allocated for grant mapping into
instead of ballooning out real RAM pages.
TODO: Fallback to real RAM pages if xen_alloc_unpopulated_dma_pages()
fails.
Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
---
drivers/xen/grant-table.c | 27 +++++++++++++++++++++++++++
1 file changed, 27 insertions(+)
diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
index 8ccccac..2bb4392 100644
--- a/drivers/xen/grant-table.c
+++ b/drivers/xen/grant-table.c
@@ -864,6 +864,25 @@ EXPORT_SYMBOL_GPL(gnttab_free_pages);
*/
int gnttab_dma_alloc_pages(struct gnttab_dma_alloc_args *args)
{
+#ifdef CONFIG_XEN_UNPOPULATED_ALLOC
+ int ret;
If we are
going to use #ifdef, then I would #ifdef the entire function, rather
than just the body. Otherwise within the function body we can use
IS_ENABLED.
+ ret = xen_alloc_unpopulated_dma_pages(args->dev, args->nr_pages,it should xen_free_unpopulated_dma_pages ?
+ args->pages);
+ if (ret < 0)
+ return ret;
+
+ ret = gnttab_pages_set_private(args->nr_pages, args->pages);
+ if (ret < 0) {
+ gnttab_dma_free_pages(args);
+ return ret;There are two things to note here.
+ }
+
+ args->vaddr = page_to_virt(args->pages[0]);
+ args->dev_bus_addr = page_to_phys(args->pages[0]);
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.
The 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.
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.
So 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.
But 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
*/
--
+ return ret;
+#else
unsigned long pfn, start_pfn;
size_t size;
int i, ret;
@@ -910,6 +929,7 @@ int gnttab_dma_alloc_pages(struct gnttab_dma_alloc_args *args)
fail:
gnttab_dma_free_pages(args);
return ret;
+#endif
}
EXPORT_SYMBOL_GPL(gnttab_dma_alloc_pages);
@@ -919,6 +939,12 @@ EXPORT_SYMBOL_GPL(gnttab_dma_alloc_pages);
*/
int gnttab_dma_free_pages(struct gnttab_dma_alloc_args *args)
{
+#ifdef CONFIG_XEN_UNPOPULATED_ALLOC
+ gnttab_pages_clear_private(args->nr_pages, args->pages);
+ xen_free_unpopulated_dma_pages(args->dev, args->nr_pages, args->pages);
+
+ return 0;
+#else
size_t size;
int i, ret;
@@ -946,6 +972,7 @@ int gnttab_dma_free_pages(struct gnttab_dma_alloc_args *args)
dma_free_wc(args->dev, size,
args->vaddr, args->dev_bus_addr);
return ret;
+#endif
}
EXPORT_SYMBOL_GPL(gnttab_dma_free_pages);
#endif
--
2.7.4