Subject: Re: [Xen-devel] [PATCH 4/7] xen/9pfs: connect to the backend
To: Stefano Stabellini <sstabellini@kernel.org>
References: <alpine.DEB.2.10.1703061147250.17906@sstabellini-ThinkPad-X260>
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 <ab7aa6b0-0bad-11e3-65a3-3493eaec07da@arm.com>
 <alpine.DEB.2.10.1703071613530.17906@sstabellini-ThinkPad-X260>
Cc: xen-devel@lists.xenproject.org, jgross@suse.com,
        Latchesar Ionkov <lucho@ionkov.net>,
        Eric Van Hensbergen <ericvh@gmail.com>, linux-kernel@vger.kernel.org,
        Stefano Stabellini <stefano@aporeto.com>,
        v9fs-developer@lists.sourceforge.net,
        Ron Minnich <rminnich@sandia.gov>, boris.ostrovsky@oracle.com
From: Julien Grall <julien.grall@arm.com>
Message-ID: <28e5fcdb-8943-d059-ae9b-d1a73e40297f@arm.com>
Date: Wed, 8 Mar 2017 12:25:37 +0000
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Hi Stefano,

On 08/03/17 00:49, Stefano Stabellini wrote:
> On Tue, 7 Mar 2017, Julien Grall wrote:
>> On 03/06/2017 08:01 PM, Stefano Stabellini wrote:
>>> +	if (ring->bytes == NULL)
>>> +		goto error;
>>> +	for (i = 0; i < (1 << XEN_9PFS_RING_ORDER); i++)
>>> +		ring->intf->ref[i] =
>>> gnttab_grant_foreign_access(dev->otherend_id,
>>> pfn_to_gfn(virt_to_pfn((void*)ring->bytes) + i), 0);.
>>
>> Please use virt_to_gfn rather than pfn_to_gfn(virt_to_pfn).
>
> OK
>
>
>> Also, this is not going to work on 64K kernel because you will grant access to
>> noncontiguous memory (e.g 0-4K, 64K-68K,...).
>
> By using virt_to_gfn like you suggested, the loop will correctly iterate
> on a 4K by 4K basis, even on a 64K kernel:
>
>   ring->bytes = (void*)__get_free_pages(GFP_KERNEL | __GFP_ZERO,
>           XEN_9PFS_RING_ORDER - (PAGE_SHIFT - XEN_PAGE_SHIFT));
>   for (i = 0; i < (1 << XEN_9PFS_RING_ORDER); i++)
>       ring->intf->ref[i] = gnttab_grant_foreign_access(dev->otherend_id, virt_to_gfn((void*)ring->bytes) + i, 0);

BTW, the cast (void *) is not necessary.

>
> where XEN_9PFS_RING_ORDER specifies the order at 4K granularity. Am I
> missing something?

I think it is fine. You could move virt_to_gfn(...) outside and avoid to 
do the translation everytime.

Cheers,

-- 
Julien Grall