On Mon, Mar 14, 2016 at 01:13:24PM +0200, Michael S. Tsirkin wrote: > On Thu, Mar 03, 2016 at 03:37:37PM +0000, Stefan Hajnoczi wrote: > > Michael pointed out that the virtio-vsock draft specification does not > > address live migration and in fact currently precludes migration. > > > > Migration is fundamental so the device specification at least mustn't > > preclude it. Having brainstormed migration with Matthew Benjamin and > > Michael Tsirkin, I am now summarizing the approach that I want to > > include in the next draft specification. > > > > Feedback and comments welcome! In the meantime I will implement this in > > code and update the draft specification. > > Most of the issue seems to be a consequence of using a 4 byte CID. > > I think the right thing to do is just to teach guests > about 64 bit CIDs. > > For now, can we drop guest CID from guest to host communication completely, > making CID only host-visible? Maybe leave the space > in the packet so we can add CID there later. > It seems that in theory this will allow changing CID > during migration, transparently to the guest. > > Guest visible CID is required for guest to guest communication - > but IIUC that is not currently supported. > Maybe that can be made conditional on 64 bit addressing. > Alternatively, it seems much easier to accept that these channels get broken > across migration. I reached the conclusion that channels break across migration because: 1. 32-bit CIDs are in sockaddr_vm and we'd break AF_VSOCK ABI by changing it to 64-bit. Application code would be specific virtio-vsock and wouldn't work with other AF_VSOCK transports that use the 32-bit sockaddr_vm struct. 2. Dropping guest CIDs from the protocol breaks network protocols that send addresses. NFS and netperf are the first two protocols I looked at and both transmit address information across the connection...
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