Re: [PATCH] virtio: 9p: correctly pass physical address to userspace for high pages

From: Rusty Russell
Date: Wed Oct 17 2012 - 23:18:39 EST

Will Deacon <will.deacon@xxxxxxx> writes:
> When using a virtio transport, the 9p net device allocates pages to back
> the descriptors inserted into the virtqueue. These allocations may be
> performed from atomic context (under the channel lock) and can therefore
> return high mappings which aren't suitable for virt_to_phys.

I had not appreciated that subtlety about GFP_ATOMIC :(

This isn't just 9p, the console, block, scsi and net devices also use

> @@ -165,7 +166,8 @@ static int vring_add_indirect(struct vring_virtqueue *vq,
> /* Use a single buffer which doesn't continue */
> head = vq->free_head;
> vq->vring.desc[head].flags = VRING_DESC_F_INDIRECT;
> - vq->vring.desc[head].addr = virt_to_phys(desc);
> + vq->vring.desc[head].addr = page_to_phys(kmap_to_page(desc)) +
> + ((unsigned long)desc & ~PAGE_MASK);
> vq->vring.desc[head].len = i * sizeof(struct vring_desc);

Gah, virt_to_phys_harder()?

What's the performance effect? If it's negligible, why doesn't
virt_to_phys() just do this for us?

We do have an alternate solution: masking out __GFP_HIGHMEM from the
kmalloc of desc. If it fails, we will fall back to laying out the
virtio request directly inside the ring; if it doesn't fit, we'll wait
for the device to consume more buffers.

> @@ -325,7 +326,7 @@ static int p9_get_mapped_pages(struct virtio_chan *chan,
> int count = nr_pages;
> while (nr_pages) {
> s = rest_of_page(data);
> - pages[index++] = virt_to_page(data);
> + pages[index++] = kmap_to_page(data);
> data += s;
> nr_pages--;
> }

This seems like a separate bug fix.

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