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

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Message-ID: <6e33b687-8862-d208-a707-77a95c61525e@gmail.com>
Date:   Sat, 8 Oct 2022 14:08:25 +0300
From:   Xenia Ragiadakou <burzalodowa@...il.com>
To:     Oleksandr Tyshchenko <olekstysh@...il.com>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Cc:     Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Juergen Gross <jgross@...e.com>
Subject: Re: [PATCH] xen/virtio: Handle cases when page offset > PAGE_SIZE
 properly


On 10/7/22 16:27, Oleksandr Tyshchenko wrote:

Hi Oleksandr

> 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>
> ---
> Cc: Juergen Gross <jgross@...e.com>
> Cc: Xenia Ragiadakou <burzalodowa@...il.com>
> 
> Depens on:
> https://lore.kernel.org/xen-devel/20221005174823.1800761-1-olekstysh@gmail.com/
> 
> Should go in only after that series.
> ---
>   drivers/xen/grant-dma-ops.c | 8 +++++---
>   1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/xen/grant-dma-ops.c b/drivers/xen/grant-dma-ops.c
> index c66f56d24013..1385f0e686fe 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),
> +			gfn_offset = PFN_DOWN(offset);
> +	unsigned int i, n_pages = PFN_UP(dma_offset + size);

IIUC, the above with a later patch will become:

dma_offset = xen_offset_in_page(offset)
gfn_offset = XEN_PFN_DOWN(offset)
n_pages = XEN_PFN_UP(dma_offset + size)

>   	grant_ref_t grant;
>   	dma_addr_t dma_handle;
>   
> @@ -187,10 +189,10 @@ 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);
> +				xen_page_to_gfn(page) + i + gfn_offset, dir == DMA_TO_DEVICE);

Here, why the pfn is not calculated before passing it to pfn_to_gfn()?
I mean sth like pfn_to_gfn(page_to_xen_pfn(page) + gfn_offset + i)

>   	}
>   
> -	dma_handle = grant_to_dma(grant) + offset;
> +	dma_handle = grant_to_dma(grant) + dma_offset;
>   
>   	return dma_handle;
>   }

-- 
Xenia