Re: [PATCH] net: tun: fix tun_xdp_one() for IFF_TUN mode

[David Woodhouse <dwmw2@infradead.org>]

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On Mon, 2021-06-21 at 15:00 +0800, Jason Wang wrote:
> I think it's probably too late to fix? Since it should work before 
> 043d222f93ab.
> 
> The only way is to backport this fix to stable.

Yeah, I assumed the fix would be backported; if not then the "does the
kernel have it" check is fairly trivial.

I *can* avoid it for now by just using TUNSNDBUF to reduce the sndbuf
and then we never take the XDP path at all.

My initial crappy hacks are slowly turning into something that I might
actually want to commit to mainline (once I've fixed endianness and
memory ordering issues):
https://gitlab.com/openconnect/openconnect/-/compare/master...vhost

I have a couple of remaining problems using vhost-net directly from
userspace though.

Firstly, I don't think I can set IFF_VNET_HDR on the tun device after
opening it. So my model of "open the tun device, then *see* if we can
use vhost to accelerate it" doesn't work.

I tried setting VHOST_NET_F_VIRTIO_NET_HDR in the vhost features
instead, but that gives me a weird failure mode where it drops around
half the incoming packets, and I haven't yet worked out why.

Of course I don't *actually* want a vnet header at all but the vhost
code really assumes that *someone* will add one; if I *don't* set
VHOST_NET_F_VIRTIO_NET_HDR then it always *assumes* it can read ten
bytes more from the tun socket than the 'peek' says, and barfs when it
can't. (Or such was my initial half-thought-through diagnosis before I
made it go away by setting IFF_VNET_HDR, at least).


Secondly, I need to pull numbers out of my posterior for the
VHOST_SET_MEM_TABLE call. This works for x86_64:

	vmem->nregions = 1;
	vmem->regions[0].guest_phys_addr = 4096;
	vmem->regions[0].memory_size = 0x7fffffffe000;
	vmem->regions[0].userspace_addr = 4096;
	if (ioctl(vpninfo->vhost_fd, VHOST_SET_MEM_TABLE, vmem) < 0) {

Is there a way to bypass that and just unconditionally set a 1:1
mapping of *all* userspace address space? 



It's possible that one or the other of those problems will result in a
new advertised "feature" which is so simple (like a 1:1 map) that we
can call it a bugfix and backport it along with the tun fix I already
posted, and the presence of *that* can indicate that the tun bug is
fixed :)

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