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. 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? 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
