Re: Designing a safe RX-zero-copy Memory Model for Networking

On Mon, Dec 12, 2016 at 06:49:03AM -0800, John Fastabend wrote:

On 16-12-12 06:14 AM, Mike Rapoport wrote:

quoted

quoted

We were not considered using XDP yet, so we've decided to limit the initial
implementation to macvtap because we can ensure correspondence between a
NIC queue and virtual NIC, which is not the case with more generic tap
device. It could be that use of XDP will allow for a generic solution for
virtio case as well.

Interesting this was one of the original ideas behind the macvlan
offload mode. iirc Vlad also was interested in this.

I'm guessing this was used because of the ability to push macvlan onto
its own queue?

Yes, with a queue dedicated to a virtual NIC we only need to ensure that
guest memory is used for RX buffers. 
 

quoted

quoted

quoted

Have you considered using "push" model for setting the NIC's RX memory?

I don't understand what you mean by a "push" model?

Currently, memory allocation in NIC drivers boils down to alloc_page with
some wrapping code. I see two possible ways to make NIC use of some
preallocated pages: either NIC driver will call an API (probably different
from alloc_page) to obtain that memory, or there will be NDO API that
allows to set the NIC's RX buffers. I named the later case "push".

I prefer the ndo op. This matches up well with AF_PACKET model where we
have "slots" and offload is just a transparent "push" of these "slots"
to the driver. Below we have a snippet of our proposed API,

(https://patchwork.ozlabs.org/patch/396714/ note the descriptor mapping
bits will be dropped)

+ * int (*ndo_direct_qpair_page_map) (struct vm_area_struct *vma,
+ *				     struct net_device *dev)
+ *	Called to map queue pair range from split_queue_pairs into
+ *	mmap region.
+

quoted

+
+static int
+ixgbe_ndo_qpair_page_map(struct vm_area_struct *vma, struct net_device *dev)
+{
+	struct ixgbe_adapter *adapter = netdev_priv(dev);
+	phys_addr_t phy_addr = pci_resource_start(adapter->pdev, 0);
+	unsigned long pfn_rx = (phy_addr + RX_DESC_ADDR_OFFSET) >> PAGE_SHIFT;
+	unsigned long pfn_tx = (phy_addr + TX_DESC_ADDR_OFFSET) >> PAGE_SHIFT;
+	unsigned long dummy_page_phy;
+	pgprot_t pre_vm_page_prot;
+	unsigned long start;
+	unsigned int i;
+	int err;
+
+	if (!dummy_page_buf) {
+		dummy_page_buf = kzalloc(PAGE_SIZE_4K, GFP_KERNEL);
+		if (!dummy_page_buf)
+			return -ENOMEM;
+
+		for (i = 0; i < PAGE_SIZE_4K / sizeof(unsigned int); i++)
+			dummy_page_buf[i] = 0xdeadbeef;
+	}
+
+	dummy_page_phy = virt_to_phys(dummy_page_buf);
+	pre_vm_page_prot = vma->vm_page_prot;
+	vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
+
+	/* assume the vm_start is 4K aligned address */
+	for (start = vma->vm_start;
+	     start < vma->vm_end;
+	     start += PAGE_SIZE_4K) {
+		if (start == vma->vm_start + RX_DESC_ADDR_OFFSET) {
+			err = remap_pfn_range(vma, start, pfn_rx, PAGE_SIZE_4K,
+					      vma->vm_page_prot);
+			if (err)
+				return -EAGAIN;
+		} else if (start == vma->vm_start + TX_DESC_ADDR_OFFSET) {
+			err = remap_pfn_range(vma, start, pfn_tx, PAGE_SIZE_4K,
+					      vma->vm_page_prot);
+			if (err)
+				return -EAGAIN;
+		} else {
+			unsigned long addr = dummy_page_phy > PAGE_SHIFT;
+
+			err = remap_pfn_range(vma, start, addr, PAGE_SIZE_4K,
+					      pre_vm_page_prot);
+			if (err)
+				return -EAGAIN;
+		}
+	}
+	return 0;
+}
+

Any thoughts on something like the above? We could push it when net-next
opens. One piece that fits naturally into vhost/macvtap is the kicks and
queue splicing are already there so no need to implement this making the
above patch much simpler.

Sorry, but I don't quite follow you here. The vhost does not use vma
mappings, it just sees a bunch of pages pointed by the vring descriptors...
 

.John

 
--
Sincerely yours,
Mike.

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>