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

[Stefano Stabellini <sstabellini@kernel.org>]

On Tue, 7 Mar 2017, Julien Grall wrote:
> Hi Stefano,
> 
> On 03/06/2017 08:01 PM, Stefano Stabellini wrote:
> > +static int xen_9pfs_front_alloc_dataring(struct xenbus_device *dev,
> > +		struct xen_9pfs_dataring *ring)
> > +{
> > +	int i;
> > +	int ret = -ENOMEM;
> > +
> > +	init_waitqueue_head(&ring->wq);
> > +	spin_lock_init(&ring->lock);
> > +	INIT_WORK(&ring->work, p9_xen_response);
> > +
> > +	ring->intf = (struct xen_9pfs_data_intf *) __get_free_page(GFP_KERNEL
> > | __GFP_ZERO);
> > +	if (!ring->intf)
> > +		goto error;
> > +	memset(ring->intf, 0, XEN_PAGE_SIZE);
> > +	ring->bytes = (void*)__get_free_pages(GFP_KERNEL | __GFP_ZERO,
> > XEN_9PFS_RING_ORDER);
> 
> The ring order will be in term of Xen page size and not Linux. So you are
> going to allocate much more memory than expected on 64KB kernel.

I'll fix.


> > +	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);

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


> We have various helper to break-down the page for you, see
> gnttab_for_one_grant, gnttab_foreach_grant, gnttab_count_grant,
> xen_for_each_gfn (though this one it is internal to xlate_mmu.c so far)
> 
> Please use them to avoid any further.
>
> > +	ring->ref = gnttab_grant_foreign_access(dev->otherend_id,
> > pfn_to_gfn(virt_to_pfn((void*)ring->intf)), 0);
> 
> Please use virt_to_gfn rather than pfn_to_gfn(virt_to_pfn).

Sure


> > +	ring->ring.in = ring->bytes;
> > +	ring->ring.out = ring->bytes + XEN_9PFS_RING_SIZE;
> > +
> > +	ret = xenbus_alloc_evtchn(dev, &ring->evtchn);
> > +	if (ret)
> > +		goto error;
> > +	ring->irq = bind_evtchn_to_irqhandler(ring->evtchn,
> > xen_9pfs_front_event_handler,
> > +					0, "xen_9pfs-frontend", ring);
> > +	if (ring->irq < 0) {
> > +		xenbus_free_evtchn(dev, ring->evtchn);
> > +		ret = ring->irq;
> > +		goto error;
> > +	}
> >  	return 0;
> > +
> > +error:
> > +	if (ring->intf != NULL)
> > +		kfree(ring->intf);
> > +	if (ring->bytes != NULL)
> > +		kfree(ring->bytes);
> > +	return ret;
> >  }