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Subject: Re: [PATCH 3/8] xen/grant-table: Allow allocating buffers suitable
 for DMA
To:     Oleksandr Andrushchenko <andr2000@gmail.com>,
        xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org,
        dri-devel@lists.freedesktop.org, linux-media@vger.kernel.org,
        jgross@suse.com, konrad.wilk@oracle.com
Cc:     daniel.vetter@intel.com, dongwon.kim@intel.com,
        matthew.d.roper@intel.com,
        Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
References: <20180525153331.31188-1-andr2000@gmail.com>
 <20180525153331.31188-4-andr2000@gmail.com>
From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
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Date:   Tue, 29 May 2018 15:10:08 -0400
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Sender: linux-kernel-owner@vger.kernel.org
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On 05/25/2018 11:33 AM, Oleksandr Andrushchenko wrote:
> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
>
> 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@epam.com>
> ---
>  drivers/xen/Kconfig       |  13 ++++
>  drivers/xen/grant-table.c | 124 ++++++++++++++++++++++++++++++++++++++
>  include/xen/grant_table.h |  25 ++++++++
>  3 files changed, 162 insertions(+)
>
> diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
> index e5d0c28372ea..3431fe210624 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


Should it depend on anything from DMA? CONFIG_HAS_DMA for example?

> +	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 d7488226e1f2..06fe6e7f639c 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 +815,82 @@ int gnttab_alloc_pages(int nr_pages, struct page **pages)
>  }
>  EXPORT_SYMBOL(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;
> +	xen_pfn_t *frames;
> +	size_t size;
> +	int i, ret;
> +
> +	frames = kcalloc(args->nr_pages, sizeof(*frames), GFP_KERNEL);
> +	if (!frames)
> +		return -ENOMEM;
> +
> +	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);
> +		ret = -ENOMEM;
> +		goto fail_free_frames;
> +	}
> +
> +	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;
> +		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, frames);
> +	if (ret != args->nr_pages) {
> +		pr_err("Failed to decrease reservation for DMA buffer\n");
> +		xenmem_reservation_increase(ret, frames);
> +		ret = -EFAULT;
> +		goto fail_free_dma;
> +	}
> +
> +	ret = gnttab_pages_set_private(args->nr_pages, args->pages);
> +	if (ret < 0)
> +		goto fail_clear_private;
> +
> +	kfree(frames);
> +	return 0;
> +
> +fail_clear_private:
> +	gnttab_pages_clear_private(args->nr_pages, args->pages);
> +fail_free_dma:


Do you need to xenmem_reservation_increase()?

> +	xenmem_reservation_va_mapping_update(args->nr_pages, args->pages,
> +					     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);
> +fail_free_frames:
> +	kfree(frames);
> +	return ret;
> +}
> +EXPORT_SYMBOL(gnttab_dma_alloc_pages);
> +#endif
> +
>  void gnttab_pages_clear_private(int nr_pages, struct page **pages)
>  {
>  	int i;
> @@ -838,6 +918,50 @@ void gnttab_free_pages(int nr_pages, struct page **pages)
>  }
>  EXPORT_SYMBOL(gnttab_free_pages);
>  
> +#ifdef CONFIG_XEN_GRANT_DMA_ALLOC
> +/**
> + * gnttab_dma_free_pages - free DMAable pages
> + * @args: arguments to the function
> + */
> +int gnttab_dma_free_pages(struct gnttab_dma_alloc_args *args)
> +{
> +	xen_pfn_t *frames;
> +	size_t size;
> +	int i, ret;
> +
> +	gnttab_pages_clear_private(args->nr_pages, args->pages);
> +
> +	frames = kcalloc(args->nr_pages, sizeof(*frames), GFP_KERNEL);


Any way you can do it without allocating memory? One possibility is to
keep allocated frames from gnttab_dma_alloc_pages(). (Not sure I like
that either but it's the only thing I can think of).


> +	if (!frames)
> +		return -ENOMEM;
> +
> +	for (i = 0; i < args->nr_pages; i++)
> +		frames[i] = page_to_xen_pfn(args->pages[i]);


Not xen_page_to_gfn()?


> +
> +	ret = xenmem_reservation_increase(args->nr_pages, 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,
> +					     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);
> +	kfree(frames);
> +	return ret;
> +}
> +EXPORT_SYMBOL(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..982e34242b9c 100644
> --- a/include/xen/grant_table.h
> +++ b/include/xen/grant_table.h
> @@ -198,6 +198,31 @@ 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.
> +	 */

Single-line comment

> +	bool coherent;
> +	/*
> +	 * Number of entries in the @pages array, defines the size
> +	 * of the DMA buffer.
> +	 */

This can be made into single line comment as well. However, I am not
sure this comment as well as those below are necessary. Fields names are
self-describing IMO.


-boris

> +	int nr_pages;
> +	/* Array of pages @pages filled with pages of the DMA buffer. */
> +	struct page **pages;
> +	/* Virtual/CPU address of the DMA buffer. */
> +	void *vaddr;
> +	/* Bus address of the DMA buffer. */
> +	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);
>