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

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Date:   Wed, 10 Nov 2021 09:34:32 +0000
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     "Wang, Wei W" <wei.w.wang@...el.com>
Cc:     "sgarzare@...hat.com" <sgarzare@...hat.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "mst@...hat.com" <mst@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "kys@...rosoft.com" <kys@...rosoft.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        "Kleen, Andi" <andi.kleen@...el.com>,
        Andra Paraschiv <andraprs@...zon.com>
Subject: Re: [RFC] hypercall-vsock: add a new vsock transport

On Wed, Nov 10, 2021 at 07:12:36AM +0000, Wang, Wei W wrote:
> 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.
> 
> 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).
> 
> 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).

AF_VSOCK is designed to allow multiple transports, so why not. There is
a cost to developing and maintaining a vsock transport though.

I think Amazon Nitro enclaves use virtio-vsock and I've CCed Andra in
case she has thoughts on the pros/cons and how to minimize the trusted
computing base.

If simplicity is the top priority then VIRTIO's MMIO transport without
indirect descriptors and using the packed virtqueue layout reduces the
size of the implementation:
https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virtio-v1.1-cs01.html#x1-1440002

Stefan

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