linux-kernel - Re: [PATCH V2 1/2] xen/virtio: Handle cases when page offset > PAGE

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Date:   Thu, 13 Oct 2022 17:22:54 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Oleksandr Tyshchenko <olekstysh@...il.com>
cc:     xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Juergen Gross <jgross@...e.com>,
        Xenia Ragiadakou <burzalodowa@...il.com>
Subject: Re: [PATCH V2 1/2] xen/virtio: Handle cases when page offset >
 PAGE_SIZE properly

On Sat, 8 Oct 2022, Oleksandr Tyshchenko wrote:
> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
> 
> 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@...m.com>

Reviewed-by: Stefano Stabellini <sstabellini@...nel.org>


> ---
> Cc: Juergen Gross <jgross@...e.com>
> Cc: Xenia Ragiadakou <burzalodowa@...il.com>
> 
> 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@gmail.com/
> ---
>  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;
>  }
> -- 
> 2.25.1
>