在 2021/6/22 下午4:29, David Woodhouse 写道:

On Tue, 2021-06-22 at 16:00 +0800, Jason Wang wrote:

quoted

quoted

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.

Easier from the kernel than from userspace though :)

Yes.

But I don't know that we need it. Isn't a call to access_ok() going to
be faster than what translate_desc() does to look things up anyway?

Right.

Using vhost_has_feature() is kind of tricky since it's used for virtio 
feature negotiation.

We probably need to use backend_features instead.

I think we should probably do more:

1) forbid the feature to be set when mem table / IOTLB has at least one 
mapping
2) forbid the mem table / IOTLB updating after the feature is set

Thanks

+               if (!access_ok((void __user *)addr, len))
+                       return -EFAULT;
+
+               iov[0].iov_len = len;
+               iov[0].iov_base = (void __user *)addr;
+               return 1;
+       }
         while ((u64)len > s) {
                 u64 size;
                 if (unlikely(ret >= iov_size)) {