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Re: [Xen-devel] [PATCH] openvswitch: Orphan frags before sending to userspace via Netlink to avoid guest stall

On 07/03/14 04:46, Pravin Shelar wrote:
On Thu, Mar 6, 2014 at 9:09 AM, Zoltan Kiss <zoltan.kiss@xxxxxxxxxx> wrote:
Do you have any feedback on this? I'm also adding KVM list as they might be
interested in this.


On 28/02/14 19:16, Zoltan Kiss wrote:

The kernel datapath now switched to zerocopy Netlink messages, but that
means that the pages on frags array are sent straight to userspace. If
pages came outside the kernel, we have to swap them out with local copies.

Signed-off-by: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>

I do not think this is required, netlink zero copy only maps
pre-allocated buffers to user-space.
How do you mean "pre-allocated"? By who?

As far as I've seen the skb in this function came straight from the device (vif in our case), and skb_zerocopy just copy the frags to user_skb, which is sent to the userspace. Those frags contain pages from guest, and it's a bad idea to pass them to userspace: e.g if userspace dies in the meantime, what happens with them? Also, in Xen's case they are actually not mapped to userspace, so accessing them can lead to garbage.


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