Re: [virtio-dev] [PATCH 0/2] introduce virtio-ism: internal shared memory device

[Xuan Zhuo <xuanzhuo@linux.alibaba.com>]

On Mon, 14 Nov 2022 22:30:53 +0100, Jan Kiszka <jan.kiszka@siemens.com> wrote:
> On 18.10.22 09:32, Jan Kiszka wrote:
> > On 17.10.22 09:47, Xuan Zhuo wrote:
> >> Hello everyone,
> >>
> >> # Background
> >>
> >> Nowadays, there is a common scenario to accelerate communication between
> >> different VMs and containers, including light weight virtual machine based
> >> containers. One way to achieve this is to colocate them on the same host.
> >> However, the performance of inter-VM communication through network stack is not
> >> optimal and may also waste extra CPU cycles. This scenario has been discussed
> >> many times, but still no generic solution available [1] [2] [3].
> >>
> >> With pci-ivshmem + SMC(Shared Memory Communications: [4]) based PoC[5],
> >> We found that by changing the communication channel between VMs from TCP to SMC
> >> with shared memory, we can achieve superior performance for a common
> >> socket-based application[5]:
> >>   - latency reduced by about 50%
> >>   - throughput increased by about 300%
> >>   - CPU consumption reduced by about 50%
> >>
> >> Since there is no particularly suitable shared memory management solution
> >> matches the need for SMC(See ## Comparison with existing technology), and virtio
> >> is the standard for communication in the virtualization world, we want to
> >> implement a virtio-ism device based on virtio, which can support on-demand
> >> memory sharing across VMs, containers or VM-container. To match the needs of SMC,
> >> the virtio-ism device need to support:
> >>
> >> 1. Dynamic provision: shared memory regions are dynamically allocated and
> >>    provisioned.
> >> 2. Multi-region management: the shared memory is divided into regions,
> >>    and a peer may allocate one or more regions from the same shared memory
> >>    device.
> >> 3. Permission control: The permission of each region can be set seperately.
> >>
> >> # Virtio ism device
> >>
> >> ISM devices provide the ability to share memory between different guests on a
> >> host. A guest's memory got from ism device can be shared with multiple peers at
> >> the same time. This shared relationship can be dynamically created and released.
> >>
> >> The shared memory obtained from the device is divided into multiple ism regions
> >> for share. ISM device provides a mechanism to notify other ism region referrers
> >> of content update events.
> >>
> >> # Usage (SMC as example)
> >>
> >> Maybe there is one of possible use cases:
> >>
> >> 1. SMC calls the interface ism_alloc_region() of the ism driver to return the
> >>    location of a memory region in the PCI space and a token.
> >> 2. The ism driver mmap the memory region and return to SMC with the token
> >> 3. SMC passes the token to the connected peer
> >> 3. the peer calls the ism driver interface ism_attach_region(token) to
> >>    get the location of the PCI space of the shared memory
> >>
> >>
> >> # About hot plugging of the ism device
> >>
> >>    Hot plugging of devices is a heavier, possibly failed, time-consuming, and
> >>    less scalable operation. So, we don't plan to support it for now.
> >>
> >> # Comparison with existing technology
> >>
> >> ## ivshmem or ivshmem 2.0 of Qemu
> >>
> >>    1. ivshmem 1.0 is a large piece of memory that can be seen by all devices that
> >>    use this VM, so the security is not enough.
> >>
> >>    2. ivshmem 2.0 is a shared memory belonging to a VM that can be read-only by all
> >>    other VMs that use the ivshmem 2.0 shared memory device, which also does not
> >>    meet our needs in terms of security.
> >
> > This is addressed by establishing separate links between VMs (modeled
> > with separate devices). That is a trade-off between simplicity of the
> > model and convenience, for sure.
>
> BTW, simplicity can also brings security because it reduces the trusted
> code base.
>
> Another feature of ivshmem-v2 is permitting direct access to essential
> resources of the device from /unprivileged/ userspace, including to the
> event triggering registers. Is your model designed for that as well? It
> not only permits VM-to-VM, it actually makes app-to-app (in VMs) very cheap.

Yes, there are two actual application scenarios or design goals:

* As we mentioned above, docking with SMC inside the linux kernel to achieve high-speed communication.
* Virtio-ISM also has an interface below /dev. Ordinary users can also directly
  obtain the SHM region resources to achieve sharing with other APPs on other
  VMs.

https://github.com/fengidri/linux-kernel-virtio-ism/commit/55a8ed21344e26f574dd81b0213b0d61d80e2ecb
https://github.com/fengidri/linux-kernel-virtio-ism/commit/6518739f9a9a36f25d5709da940b7a7938f8e0ee

Thanks.

>
> Jan
>
> --
> Siemens AG, Technology
> Competence Center Embedded Linux
>