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Subject: Re: [PATCH 3/8] xen/grant-table: Allow allocating buffers suitable
 for DMA
To:     Boris Ostrovsky <boris.ostrovsky@oracle.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>
 <94de6bd7-405c-c43f-0468-be71efff7552@oracle.com>
From:   Oleksandr Andrushchenko <andr2000@gmail.com>
Message-ID: <c2f9f6b4-03bd-225b-a42d-b071958dd899@gmail.com>
Date:   Wed, 30 May 2018 09:34:44 +0300
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On 05/29/2018 10:10 PM, Boris Ostrovsky wrote:
> 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?
Yes, it must be "depends on XEN && HAS_DMA",
thank you
>
>> +	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()?
Yes, missed that on fail_clear_private error path, will fix
>
>> +	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).
Yes, I was also thinking about storing the allocated frames array from
gnttab_dma_alloc_pages(), but that seemed not to be clear enough as
the caller of the gnttab_dma_alloc_pages will need to store those frames
in some context, so we can pass them on free. But the caller doesn't really
need the frames which might confuse, so I decided to make those allocations
on the fly.
But I can still rework that to store the frames if you insist: please 
let me know.
>
>
>> +	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()?
Well, according to [1] it should be :
     /* XENMEM_populate_physmap requires a PFN based on Xen
      * granularity.
      */
     frame_list[i] = page_to_xen_pfn(page);

>
>> +
>> +	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
will fix
>
>> +	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.
will remove
>
> -boris
Thank you,
Oleksandr
>> +	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);
>>   
[1] 
https://elixir.bootlin.com/linux/v4.17-rc7/source/drivers/xen/balloon.c#L485