Re: [RFC PATCH 2/2] xen/grant-table: Use unpopulated DMAable pages instead of real RAM ones

[Oleksandr <olekstysh@gmail.com>]

On 11.06.22 02:55, Stefano Stabellini wrote:

Hello Stefano

> On Thu, 9 Jun 2022, Oleksandr wrote:
>> On 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:
>>>> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
>>>>
>>>> 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@epam.com>
>>>> ---
>>>>    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;
>>> This is an alternative implementation of the same function.
>> Currently, yes.
>>
>>
>>>    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.
>>
>> Good point. Note, there is one missing thing in current patch which is
>> described in TODO.
>>
>> "Fallback to real RAM pages if xen_alloc_unpopulated_dma_pages() fails."  So I
>> will likely use IS_ENABLED within the function body.
>>
>> If CONFIG_XEN_UNPOPULATED_ALLOC is enabled then gnttab_dma_alloc_pages() will
>> try to call xen_alloc_unpopulated_dma_pages() the first and if fails then
>> fallback to allocate RAM pages and balloon them out.
>>
>> One moment is not entirely clear to me. If we use fallback in
>> gnttab_dma_alloc_pages() then we must use fallback in gnttab_dma_free_pages()
>> as well, we cannot use xen_free_unpopulated_dma_pages() for real RAM pages.
>> The question is how to pass this information to the gnttab_dma_free_pages()?
>> The first idea which comes to mind is to add a flag to struct
>> gnttab_dma_alloc_args...
>   
> You can check if the page is within the mhp_range range or part of
> iomem_resource? If not, you can free it as a normal page.
>
> If we do this, then the fallback is better implemented in
> unpopulated-alloc.c because that is the one that is aware about
> page addresses.


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_args
2. (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?


>
>   
>   
>>>> +	ret = xen_alloc_unpopulated_dma_pages(args->dev, args->nr_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);
>>> it should xen_free_unpopulated_dma_pages ?
>> Besides calling the xen_free_unpopulated_dma_pages(), we also need to call
>> gnttab_pages_clear_private() here, this is what gnttab_dma_free_pages() is
>> doing.
>>
>> I can change to call both function instead:
>>
>>      gnttab_pages_clear_private(args->nr_pages, args->pages);
>>      xen_free_unpopulated_dma_pages(args->dev, args->nr_pages, args->pages);
>>
>> Shall I?
> No, leave it as is. I didn't realize that gnttab_pages_set_private can
> fail half-way through.


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.
>> agree
>>
>>
>>>
>>> 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.
>> 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.
> Yeah
>   
>
>>> 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.
>> agree
>>
>>
>>> 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
>>>    */
>> 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?
>   
> In that case, an IOMMU is required. If an IOMMU is setup correct, then
> 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.


>   
>   
>   
>>>> +	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

-- 
Regards,

Oleksandr Tyshchenko