Re: [PATCH 2/2] virtio_net: Improve the recv buffer allocation scheme

From: Anthony Liguori
Date: Thu Oct 09 2008 - 15:26:48 EST


Mark McLoughlin wrote:

Also, including virtio_net_hdr in the data buffer would need another
feature flag. Rightly or wrongly, KVM's implementation requires
virtio_net_hdr to be the first buffer:

if (elem.in_num < 1 || elem.in_sg[0].iov_len != sizeof(*hdr)) {
fprintf(stderr, "virtio-net header not in first element\n");
exit(1);
}

i.e. it's part of the ABI ... at least as KVM sees it :-)

This is actually something that's broken in a nasty way. Having the header in the first element is not supposed to be part of the ABI but it sort of has to be ATM.

If an older version of QEMU were to use a newer kernel, and the newer kernel had a larger header size, then if we just made the header be the first X bytes, QEMU has no way of knowing how many bytes that should be. Instead, the guest actually has to allocate the virtio-net header in such a way that it only presents the size depending on the features that the host supports. We don't use a simple versioning scheme, so you'd have to check for a combination of features advertised by the host but that's not good enough because the host may disable certain features.

Perhaps the header size is whatever the longest element that has been commonly negotiated?

So that's why this aggressive check is here. Not to necessarily cement this into the ABI but as a way to make someone figure out how to sanitize this all.

Regards,

Anthony Liguori
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