Re: Inter-VM device emulation (call on Mon 20th July 2020)

Alex Bennée
2020-07-27T12:22:34Z

Michael S. Tsirkin <mst@redhat.com> writes:

> On Mon, Jul 27, 2020 at 11:30:24AM +0100, Alex BennÃ©e wrote:
>>
>> Stefan Hajnoczi <stefanha@redhat.com> writes:
>>
>> > On Tue, Jul 21, 2020 at 11:49:04AM +0100, Alex BennÃ©e wrote:
>> >> Stefan Hajnoczi <stefanha@gmail.com> writes:
<snip>
>> >> Another thing that came across in the call was quite a lot of
>> >> assumptions about QEMU and Linux w.r.t virtio. While our project will
>> >> likely have Linux as a guest OS we are looking specifically at enabling
>> >> virtio for Type-1 hypervisors like Xen and the various safety certified
>> >> proprietary ones. It is unlikely that QEMU would be used as the VMM for
>> >> these deployments. We want to work out what sort of common facilities
>> >> hypervisors need to support to enable virtio so the daemons can be
>> >> re-usable and maybe setup with a minimal shim for the particular
>> >> hypervisor in question.
>> >
>> > The vhost-user protocol together with the backend program conventions
>> > define the wire protocol and command-line interface (see
>> > docs/interop/vhost-user.rst).
>> >
>> > vhost-user is already used by other VMMs today. For example,
>> > cloud-hypervisor implements vhost-user.
>>
>> Ohh that's a new one for me. I see it is a KVM only project but it's
>> nice to see another VMM using the common rust-vmm backend. There is
>> interest in using rust-vmm to implement VMMs for type-1 hypervisors but
>> we need to work out if there are two many type-2 concepts backed into
>> the lower level rust crates.
>>
>> > I'm sure there is room for improvement, but it seems like an incremental
>> > step given that vhost-user already tries to cater for this scenario.
>> >
>> > Are there any specific gaps you have identified?
>>
>> Aside from the desire to limit the shared memory footprint between the
>> backend daemon and a guest not yet.
>
> So it's certainly nice for security but not really a requirement for a
> type-1 HV, right?

Not a requirement per-se but type-1 setups don't assume a "one userspace
to rule them all" approach.

>> I suspect the eventfd mechanism might just end up being simulated by the
>> VMM as a result of whatever comes from the type-1 interface indicating a
>> doorbell has been rung. It is after all just a FD you consume numbers
>> over right?
>
> Does not even have to be numbers. We need a way to be woken up, a way to
> stop/start listening for wakeups and a way to detect that there was a
> wakeup while we were not listening.
>
> Though there are special tricks for offloads where we poke through
> layers in order to map things directly to hardware.
>
>> Not all setups will have an equivalent of a Dom0 "master" guest to do
>> orchestration. Highly embedded are likely to have fixed domains created
>> as the firmware/hypervisor start up.
>>
>> >
>> > Stefan
>>
>>
>> --
>> Alex BennÃ©e

--
Alex Bennée