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

[Julien Grall]

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.

> +	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).

Also, this is not going to work on 64K kernel because you will grant 
access to noncontiguous memory (e.g 0-4K, 64K-68K,...).

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).

> +	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;
>  }

Cheers,

--
Julien Grall