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

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Date:   Wed, 8 Mar 2017 11:08:35 -0800 (PST)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Julien Grall <julien.grall@....com>
cc:     Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xenproject.org, jgross@...e.com,
        Latchesar Ionkov <lucho@...kov.net>,
        Eric Van Hensbergen <ericvh@...il.com>,
        linux-kernel@...r.kernel.org,
        Stefano Stabellini <stefano@...reto.com>,
        v9fs-developer@...ts.sourceforge.net,
        Ron Minnich <rminnich@...dia.gov>, boris.ostrovsky@...cle.com
Subject: Re: [Xen-devel] [PATCH 4/7] xen/9pfs: connect to the backend

On Wed, 8 Mar 2017, Julien Grall wrote:
> 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.

All right, thanks.