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

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