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

在 2021/6/22 下午3:28, David Woodhouse 写道:

On Tue, 2021-06-22 at 12:34 +0800, Jason Wang wrote:

quoted

quoted

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 :)

This means we need validate the userspace address each time before vhost 
tries to use that. This will de-gradate the performance. So we still 
need to figure out the legal userspace address range which might not be 
easy.

Thanks