Re: [Qemu-devel] Towards an ivshmem 2.0?

[Jan Kiszka]

On 2017-01-16 15:18, Stefan Hajnoczi wrote:
> On Mon, Jan 16, 2017 at 09:36:51AM +0100, Jan Kiszka wrote:
>> some of you may know that we are using a shared memory device similar to
>> ivshmem in the partitioning hypervisor Jailhouse [1].
>>
>> We started as being compatible to the original ivshmem that QEMU
>> implements, but we quickly deviated in some details, and in the recent
>> months even more. Some of the deviations are related to making the
>> implementation simpler. The new ivshmem takes <500 LoC - Jailhouse is
>> aiming at safety critical systems and, therefore, a small code base.
>> Other changes address deficits in the original design, like missing
>> life-cycle management.
> 
> My first thought is "what about virtio?".  Can you share some background
> on why ivshmem fits the use case better than virtio?
> 
> The reason I ask is because the ivshmem devices you define would have
> parallels to existing virtio devices and this could lead to duplication.

virtio was created as an interface between a host and a guest. It has no
notion of direct (or even symmetric) connection between guests. With
ivshmem, we want to establish only a minimal host-guest interface. We
want to keep the host out of the business negotiating protocol details
between two connected guests.

So, the trade-off was between reusing existing virtio drivers - in the
best case, some changes would have been required definitely - and
requiring complex translation of virtio into a vm-to-vm model on the one
side and establishing a new driver ecosystem on much simpler host
services (500 LoC...). We went for the latter.

Jan

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