On Mon, Jul 27, 2020 at 11:30:24AM +0100, Alex Bennée wrote: > > Stefan Hajnoczi <stefanha@xxxxxxxxxx> writes: > > > On Tue, Jul 21, 2020 at 11:49:04AM +0100, Alex Bennée wrote: > >> Stefan Hajnoczi <stefanha@xxxxxxxxx> writes: > >> > 2. Alexander Graf's idea for a new Linux driver that provides an > >> > enforcing software IOMMU. This would be a character device driver that > >> > is mmapped by the device emulation process (either vhost-user-style on > >> > the host or another VMM for inter-VM device emulation). The Driver VMM > >> > can program mappings into the device and the page tables in the device > >> > emulation process will be updated. This way the Driver VMM can share > >> > memory specific regions of guest RAM with the device emulation process > >> > and revoke those mappings later. > >> > >> I'm wondering if there is enough plumbing on the guest side so a guest > >> can use the virtio-iommu to mark out exactly which bits of memory the > >> virtual device can have access to? At a minimum the virtqueues need to > >> be accessible and for larger transfers maybe a bounce buffer. However > >> for speed you want as wide as possible mapping but no more. It would be > >> nice for example if a block device could load data directly into the > >> guests block cache (zero-copy) but without getting a view of the kernels > >> internal data structures. > > > > Maybe Jean-Philippe or Eric can answer that? > > > >> 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? > 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 Bennée
