linux-kernel - Re: [PATCH 1/2] xen/virtio: Fix n_pages calculation in xen_grant_dma_map(unmap)

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Date:   Thu, 6 Oct 2022 12:24:20 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Oleksandr Tyshchenko <olekstysh@...il.com>,
        Xenia Ragiadakou <burzalodowa@...il.com>
Cc:     xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Stefano Stabellini <sstabellini@...nel.org>
Subject: Re: [PATCH 1/2] xen/virtio: Fix n_pages calculation in
 xen_grant_dma_map(unmap)_page()

On 06.10.22 12:14, Oleksandr Tyshchenko wrote:
> 
> 
> On Thu, Oct 6, 2022 at 11:05 AM Juergen Gross <jgross@...e.com 
> <mailto:jgross@...e.com>> wrote:
> 
>     On 06.10.22 09:35, Xenia Ragiadakou wrote:
> 
> 
> 
> Hello Xenia, Juergen
> 
> [sorry for the possible format issues]
> 
>      >
>      > On 10/5/22 20:48, Oleksandr Tyshchenko wrote:
>      >> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com
>     <mailto:oleksandr_tyshchenko@...m.com>>
>      >>
>      >> Take page offset into the account when calculating the number of pages
>      >> to be granted.
>      >>
>      >> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com
>     <mailto:oleksandr_tyshchenko@...m.com>>
>      >> Fixes: d6aca3504c7d ("xen/grant-dma-ops: Add option to restrict memory
>     access
>      >> under Xen")
>      >> ---
>      >>   drivers/xen/grant-dma-ops.c | 5 +++--
>      >>   1 file changed, 3 insertions(+), 2 deletions(-)
>      >>
>      >> diff --git a/drivers/xen/grant-dma-ops.c b/drivers/xen/grant-dma-ops.c
>      >> index 8973fc1e9ccc..1998d0e8ce82 100644
>      >> --- a/drivers/xen/grant-dma-ops.c
>      >> +++ b/drivers/xen/grant-dma-ops.c
>      >> @@ -153,7 +153,7 @@ 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(size);
>      >> +    unsigned int i, n_pages = PFN_UP(offset + size);
>      >
>      > Here, why do we use PFN_UP and not XEN_PFN_UP?
>      > Also, since the variable 'n_pages' seems to refer to the number of grants
>      > (unless I got it all entirely wrong ...), wouldn't it be more suitable to
>     call
>      > explicitly gnttab_count_grant()?
> 
>     Good point.
> 
> 
> +1
> 
> 
>     I think this will need another patch for switching grant-dma-ops.c to
>     use XEN_PAGE_SIZE and XEN_PAGE_SHIFT.
> 
> 
> +1
> 
> I can create a separate patch for converting on top of this series, it would be 
> nice to clarify one point.
> 
> So I will convert PAGE_SIZE/PAGE_SHIFT to XEN_PAGE_SIZE/XEN_PAGE_SHIFT 
> respectively (where appropriate).

Yes, that would be the idea.

> Should the PFN_UP be converted to XEN_PFN_UP *or* use 
> gnttab_count_grant() explicitly? Personally I would prefer the former, but would 
> also be ok with the latter.

I agree XEN_PFN_UP would be better, especially as XEN_PAGE_SIZE/XEN_PAGE_SHIFT
will be used in the same functions.


Juergen

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