Re: [PATCH V2 1/2] xen/virtio: Handle cases when page offset > PAGE_SIZE properly

[Xenia Ragiadakou <burzalodowa@xxxxxxxxx>]

On 10/8/22 18:10, Oleksandr Tyshchenko wrote:
> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
>
> Passed to xen_grant_dma_map_page() offset in the page
> can be > PAGE_SIZE even if the guest uses the same page granularity
> as Xen (4KB).
>
> Before current patch, if such case happened we ended up providing
> grants for the whole region in xen_grant_dma_map_page() which
> was really unnecessary. The more, we ended up not releasing all
> grants which represented that region in xen_grant_dma_unmap_page().
>
> Current patch updates the code to be able to deal with such cases.
>
> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
Reviewed-by: Xenia Ragiadakou <burzalodowa@xxxxxxxxx>

> ---
> Cc: Juergen Gross <jgross@xxxxxxxx>
> Cc: Xenia Ragiadakou <burzalodowa@xxxxxxxxx>
>
> Changes V1 -> V2:
>     - s/gfn_offset/pfn_offset
>     - clarify gfn calculation when granting a page
>
> Previous discussion is at:
> https://lore.kernel.org/lkml/20221007132736.2275574-1-olekstysh@xxxxxxxxx/
> ---
>   drivers/xen/grant-dma-ops.c | 9 ++++++---
>   1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/xen/grant-dma-ops.c b/drivers/xen/grant-dma-ops.c
> index c66f56d24013..1d018e3a68a0 100644
> --- a/drivers/xen/grant-dma-ops.c
> +++ b/drivers/xen/grant-dma-ops.c
> @@ -168,7 +168,9 @@ static dma_addr_t xen_grant_dma_map_page(struct device 
> *dev, struct page *page,
>                                          unsigned long attrs)
>   {
>         struct xen_grant_dma_data *data;
> -       unsigned int i, n_pages = PFN_UP(offset + size);
> +       unsigned long dma_offset = offset_in_page(offset),
> +                       pfn_offset = PFN_DOWN(offset);
> +       unsigned int i, n_pages = PFN_UP(dma_offset + size);
>         grant_ref_t grant;
>         dma_addr_t dma_handle;
>   
> @@ -187,10 +189,11 @@ static dma_addr_t xen_grant_dma_map_page(struct device *dev, struct page *page,
>   
>   	for (i = 0; i < n_pages; i++) {
>                 gnttab_grant_foreign_access_ref(grant + i, data->backend_domid,
> -                               xen_page_to_gfn(page) + i, dir == 
> DMA_TO_DEVICE);
> +                               pfn_to_gfn(page_to_xen_pfn(page) + i + 
> pfn_offset),
> +                               dir == DMA_TO_DEVICE);
>         }
>   
> -	dma_handle = grant_to_dma(grant) + offset;
> +       dma_handle = grant_to_dma(grant) + dma_offset;
>   
>   	return dma_handle;
>   }
--
Xenia