Re: [External] Re: [PATCH 0/4] Add multi-cid support for vsock driver

[Stefano Garzarella]

On Wed, Aug 04, 2021 at 03:09:41PM +0800, 傅关丞 wrote:
> Sorry I cannot figure out a good use case.
>
> It is normal for a host to have multiple ip addresses used for
> communication.
> So I thought it might be nice to have both  host and guest use multiple
> CIDs for communication.
> I know this is not a very strong argument.

Maybe there could be a use case for guests (which I don't see now), but 
for the host it seems pointless. The strength of vsock is that the guest 
knows that using CID=2 always reaches the host.

Moreover we have recently merged VMADDR_FLAG_TO_HOST that when set 
allows you to forward any packet to the host, regardless of the CID (not 
yet supported by vhost-vsock).

> The vsock driver does not work if one of the two peers doesn't support
> multiple CIDs.

This is absolutely to be avoided.

I think the virtio device feature negotiation can help here.

> I have a possible solution here, but there may be some problems with it
> that I haven't noticed.
>
> Hypervisors will use different ways to send CIDs setup to the kernel based
> on their vsock setup.
>
> ------For host-------
> If host vsock driver supports multi-cid, the hypervisor will use the
> modified VHOST_VSOCK_SET_GUEST_CID call to set its CIDs.
> Otherwise, the original call is used.
>
> ------For guest-------
> Now the virtio_vsock_config looks like this:
> u64 guest_cid
> u32 num_guest_cid;
> u32 num_host_cid;
> u32 index;
> u64 cid;
>
> If the guest vsock driver supports multi-cid, it will read num_guest_cid
> and num_host_cid from the device config space.
> Then it writes an index register, which is the cid it wants to read.  After
> hypervisors handle this issue, it can read the cid
> from the cid register.
>
> If it does not support multi-cid, it will just read the guest_cid from the
> config space, which should work just fine.

Why not add a new device feature to enable or disable multi-cid?


> -------Communication--------
> For communication issues, we might need to use a new feature bit.  Let's
> call it VHOST_VSOCK_SUPPORT_MULTI_CID.
> The basic idea is that this feature bit is set when both host and guest
> support using multiple CIDs.  After negotiation, if the feature bit
> is set, the host can use all the CIDs specified to communicate with the
> guest.  Otherwise, the first cid passed in will
> be used as the guest_cid to communicate with guests.

I think the same feature bit can be used for the virtio_vsock_config, 
no?

> Also, if the bit is set for guests, all the CIDs can be used to communicate
> with the host.  Otherwise, the first cid with index 0 will be
> used as the guest_cid while the VMADDR_HOST_CID will be used for host cid.

We already have VMADDR_FLAG_TO_HOST to forward all packets to the host, 
we only need to support in some way in vhost-vsock.

Thanks,
Stefano