Re: [RFC] hypercall-vsock: add a new vsock transport

[Michael S. Tsirkin]

On Wed, Nov 10, 2021 at 07:12:36AM +0000, Wang, Wei W wrote:
> Hi,
> 
>  
> 
> We plan to add a new vsock transport based on hypercall (e.g. vmcall on Intel
> CPUs).
> 
> It transports AF_VSOCK packets between the guest and host, which is similar to
> 
> virtio-vsock, vmci-vsock and hyperv-vsock.
> 
>  
> 
> Compared to the above listed vsock transports which are designed for high
> performance,
> 
> the main advantages of hypercall-vsock are:
> 
> 1)       It is VMM agnostic. For example, one guest working on hypercall-vsock
> can run on
> 
> either KVM, Hyperv, or VMware.

hypercalls are fundamentally hypervisor dependent though.
Assuming you can carve up a hypervisor independent hypercall,
using it for something as mundane and specific as vsock for TDX
seems like a huge overkill. For example, virtio could benefit from
faster vmexits that hypercalls give you for signalling.
How about a combination of virtio-mmio and hypercalls for fast-path
signalling then?

> 2)       It is simpler. It doesn’t rely on any complex bus enumeration
> 
> (e.g. virtio-pci based vsock device may need the whole implementation of PCI).
> 

Next thing people will try to do is implement a bunch of other device on
top of it.  virtio used pci simply because everyone implements pci.  And
the reason for *that* is because implementing a basic pci bus is dead
simple, whole of pci.c in qemu is <3000 LOC.

> 
> An example usage is the communication between MigTD and host (Page 8 at
> 
> https://static.sched.com/hosted_files/kvmforum2021/ef/
> TDX%20Live%20Migration_Wei%20Wang.pdf).
> 
> MigTD communicates to host to assist the migration of the target (user) TD.
> 
> MigTD is part of the TCB, so its implementation is expected to be as simple as
> possible
> 
> (e.g. bare mental implementation without OS, no PCI driver support).
> 
>  

Try to list drawbacks? For example, passthrough for nested virt
isn't possible unlike pci, neither are hardware implementations.


> Looking forward to your feedbacks.
> 
>  
> 
> Thanks,
> 
> Wei
>