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

[Rusty Russell <rusty@rustcorp.com.au>]

Will Deacon <will.deacon@arm.com> 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
GFP_ATOMIC.

> @@ -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.

Cheers,
Rusty.