Re: [PATCH v2 4/9] xen/grant-table: Allow allocating buffers suitable for DMA

[Boris Ostrovsky]

On 06/01/2018 07:41 AM, Oleksandr Andrushchenko wrote:
> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
>
> Extend grant table module API to allow allocating buffers that can
> be used for DMA operations and mapping foreign grant references
> on top of those.
> The resulting buffer is similar to the one allocated by the balloon
> driver in terms that proper memory reservation is made
> ({increase|decrease}_reservation and VA mappings updated if needed).
> This is useful for sharing foreign buffers with HW drivers which
> cannot work with scattered buffers provided by the balloon driver,
> but require DMAable memory instead.
>
> Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@xxxxxxxx>
> ---
>  drivers/xen/Kconfig       |  13 +++++
>  drivers/xen/grant-table.c | 109 ++++++++++++++++++++++++++++++++++++++
>  include/xen/grant_table.h |  18 +++++++
>  3 files changed, 140 insertions(+)
>
> diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
> index e5d0c28372ea..39536ddfbce4 100644
> --- a/drivers/xen/Kconfig
> +++ b/drivers/xen/Kconfig
> @@ -161,6 +161,19 @@ config XEN_GRANT_DEV_ALLOC
>  	  to other domains. This can be used to implement frontend drivers
>  	  or as part of an inter-domain shared memory channel.
>  
> +config XEN_GRANT_DMA_ALLOC
> +	bool "Allow allocating DMA capable buffers with grant reference module"
> +	depends on XEN && HAS_DMA
> +	help
> +	  Extends grant table module API to allow allocating DMA capable
> +	  buffers and mapping foreign grant references on top of it.
> +	  The resulting buffer is similar to one allocated by the balloon
> +	  driver in terms that proper memory reservation is made
> +	  ({increase|decrease}_reservation and VA mappings updated if needed).
> +	  This is useful for sharing foreign buffers with HW drivers which
> +	  cannot work with scattered buffers provided by the balloon driver,
> +	  but require DMAable memory instead.
> +
>  config SWIOTLB_XEN
>  	def_bool y
>  	select SWIOTLB
> diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
> index dbb48a89e987..5658e58d9cc6 100644
> --- a/drivers/xen/grant-table.c
> +++ b/drivers/xen/grant-table.c
> @@ -45,6 +45,9 @@
>  #include <linux/workqueue.h>
>  #include <linux/ratelimit.h>
>  #include <linux/moduleparam.h>
> +#ifdef CONFIG_XEN_GRANT_DMA_ALLOC
> +#include <linux/dma-mapping.h>
> +#endif
>  
>  #include <xen/xen.h>
>  #include <xen/interface/xen.h>
> @@ -57,6 +60,7 @@
>  #ifdef CONFIG_X86
>  #include <asm/xen/cpuid.h>
>  #endif
> +#include <xen/mem-reservation.h>
>  #include <asm/xen/hypercall.h>
>  #include <asm/xen/interface.h>
>  
> @@ -811,6 +815,73 @@ int gnttab_alloc_pages(int nr_pages, struct page **pages)
>  }
>  EXPORT_SYMBOL_GPL(gnttab_alloc_pages);
>  
> +#ifdef CONFIG_XEN_GRANT_DMA_ALLOC
> +/**
> + * gnttab_dma_alloc_pages - alloc DMAable pages suitable for grant mapping into
> + * @args: arguments to the function
> + */
> +int gnttab_dma_alloc_pages(struct gnttab_dma_alloc_args *args)
> +{
> +	unsigned long pfn, start_pfn;
> +	size_t size;
> +	int i, ret;
> +
> +	size = args->nr_pages << PAGE_SHIFT;
> +	if (args->coherent)
> +		args->vaddr = dma_alloc_coherent(args->dev, size,
> +						 &args->dev_bus_addr,
> +						 GFP_KERNEL | __GFP_NOWARN);
> +	else
> +		args->vaddr = dma_alloc_wc(args->dev, size,
> +					   &args->dev_bus_addr,
> +					   GFP_KERNEL | __GFP_NOWARN);
> +	if (!args->vaddr) {
> +		pr_err("Failed to allocate DMA buffer of size %zu\n", size);
> +		return -ENOMEM;
> +	}
> +
> +	start_pfn = __phys_to_pfn(args->dev_bus_addr);
> +	for (pfn = start_pfn, i = 0; pfn < start_pfn + args->nr_pages;
> +			pfn++, i++) {
> +		struct page *page = pfn_to_page(pfn);
> +
> +		args->pages[i] = page;
> +		args->frames[i] = xen_page_to_gfn(page);
> +		xenmem_reservation_scrub_page(page);
> +	}
> +
> +	xenmem_reservation_va_mapping_reset(args->nr_pages, args->pages);
> +
> +	ret = xenmem_reservation_decrease(args->nr_pages, args->frames);
> +	if (ret != args->nr_pages) {
> +		pr_err("Failed to decrease reservation for DMA buffer\n");
> +		ret = -EFAULT;
> +		goto fail_free_dma;
> +	}
> +
> +	ret = gnttab_pages_set_private(args->nr_pages, args->pages);
> +	if (ret < 0)
> +		goto fail_clear_private;
> +
> +	return 0;
> +
> +fail_clear_private:
> +	gnttab_pages_clear_private(args->nr_pages, args->pages);
> +fail_free_dma:
> +	xenmem_reservation_increase(args->nr_pages, args->frames);
> +	xenmem_reservation_va_mapping_update(args->nr_pages, args->pages,
> +					     args->frames);
> +	if (args->coherent)
> +		dma_free_coherent(args->dev, size,
> +				  args->vaddr, args->dev_bus_addr);
> +	else
> +		dma_free_wc(args->dev, size,
> +			    args->vaddr, args->dev_bus_addr);
> +	return ret;
> +}


Would it be possible to call gnttab_dma_free_pages() here?


> +EXPORT_SYMBOL_GPL(gnttab_dma_alloc_pages);
> +#endif
> +
>  void gnttab_pages_clear_private(int nr_pages, struct page **pages)
>  {
>  	int i;
> @@ -838,6 +909,44 @@ void gnttab_free_pages(int nr_pages, struct page **pages)
>  }
>  EXPORT_SYMBOL_GPL(gnttab_free_pages);
>  
> +#ifdef CONFIG_XEN_GRANT_DMA_ALLOC

I'd move this after (or before) gnttab_dma_alloc_page() to keep both
inside a single ifdef block.

-boris


> +/**
> + * gnttab_dma_free_pages - free DMAable pages
> + * @args: arguments to the function
> + */
> +int gnttab_dma_free_pages(struct gnttab_dma_alloc_args *args)
> +{
> +	size_t size;
> +	int i, ret;
> +
> +	gnttab_pages_clear_private(args->nr_pages, args->pages);
> +
> +	for (i = 0; i < args->nr_pages; i++)
> +		args->frames[i] = page_to_xen_pfn(args->pages[i]);
> +
> +	ret = xenmem_reservation_increase(args->nr_pages, args->frames);
> +	if (ret != args->nr_pages) {
> +		pr_err("Failed to decrease reservation for DMA buffer\n");
> +		ret = -EFAULT;
> +	} else {
> +		ret = 0;
> +	}
> +
> +	xenmem_reservation_va_mapping_update(args->nr_pages, args->pages,
> +					     args->frames);
> +
> +	size = args->nr_pages << PAGE_SHIFT;
> +	if (args->coherent)
> +		dma_free_coherent(args->dev, size,
> +				  args->vaddr, args->dev_bus_addr);
> +	else
> +		dma_free_wc(args->dev, size,
> +			    args->vaddr, args->dev_bus_addr);
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(gnttab_dma_free_pages);
> +#endif
> +
>  /* Handling of paged out grant targets (GNTST_eagain) */
>  #define MAX_DELAY 256
>  static inline void
> diff --git a/include/xen/grant_table.h b/include/xen/grant_table.h
> index de03f2542bb7..9bc5bc07d4d3 100644
> --- a/include/xen/grant_table.h
> +++ b/include/xen/grant_table.h
> @@ -198,6 +198,24 @@ void gnttab_free_auto_xlat_frames(void);
>  int gnttab_alloc_pages(int nr_pages, struct page **pages);
>  void gnttab_free_pages(int nr_pages, struct page **pages);
>  
> +#ifdef CONFIG_XEN_GRANT_DMA_ALLOC
> +struct gnttab_dma_alloc_args {
> +	/* Device for which DMA memory will be/was allocated. */
> +	struct device *dev;
> +	/* If set then DMA buffer is coherent and write-combine otherwise. */
> +	bool coherent;
> +
> +	int nr_pages;
> +	struct page **pages;
> +	xen_pfn_t *frames;
> +	void *vaddr;
> +	dma_addr_t dev_bus_addr;
> +};
> +
> +int gnttab_dma_alloc_pages(struct gnttab_dma_alloc_args *args);
> +int gnttab_dma_free_pages(struct gnttab_dma_alloc_args *args);
> +#endif
> +
>  int gnttab_pages_set_private(int nr_pages, struct page **pages);
>  void gnttab_pages_clear_private(int nr_pages, struct page **pages);
>