netdev - Re: [PATCH] net: tun: fix tun_xdp_one() for IFF

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Date:   Tue, 22 Jun 2021 08:28:51 +0100
From:   David Woodhouse <dwmw2@...radead.org>
To:     Jason Wang <jasowang@...hat.com>, netdev <netdev@...r.kernel.org>
Cc:     Eugenio Pérez <eperezma@...hat.com>
Subject: Re: [PATCH] net: tun: fix tun_xdp_one() for IFF_TUN mode

On Tue, 2021-06-22 at 12:34 +0800, Jason Wang wrote:
> > 
> > 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?
> 
> 
> Memory Table is one of the basic abstraction of the vhost. Basically, 
> you only need to map the userspace buffers. This is how DPDK virtio-user 
> PMD did. Vhost will validate the addresses through access_ok() during 
> VHOST_SET_MEM_TABLE.
> 
> The range of all usersapce space seems architecture specific, I'm not 
> sure if it's worth to bother.

The buffers are just malloc'd. I just need a full 1:1 mapping of all
"guest" memory to userspace addresses, and was trying to avoid having
to map them on demand *just* because I don't know the full range of
possible addresses that malloc will return, in advance.

I'm tempted to add a new feature for that 1:1 access, with no ->umem or
->iotlb at all. And then I can use it as a key to know that the XDP
bugs are fixed too :)


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