Re: [Xen-devel] [PATCH 4/7] xen/9pfs: connect to the backend

[Julien Grall]

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