Jan Kiszka <jan.kis...@siemens.com> writes: > On 2014-06-12 04:27, Rusty Russell wrote: >> Henning Schild <henning.sch...@siemens.com> writes: >> It was also never implemented, and remains a thought experiment. >> However, implementing it in lguest should be fairly easy. > > The reason why a trusted helper, i.e. additional logic in the > hypervisor, is not our favorite solution is that we'd like to keep the > hypervisor as small as possible. I wouldn't exclude such an approach > categorically, but we have to weigh the costs (lines of code, additional > hypervisor interface) carefully against the gain (existing > specifications and guest driver infrastructure).
Reasonable, but I think you'll find it is about the minimal implementation in practice. Unfortunately, I don't have time during the next 6 months to implement it myself :( > Back to VIRTIO_F_RING_SHMEM_ADDR (which you once brought up in an MCA > working group discussion): What speaks against introducing an > alternative encoding of addresses inside virtio data structures? The > idea of this flag was to replace guest-physical addresses with offsets > into a shared memory region associated with or part of a virtio > device. We would also need a way of defining the shared memory region. But that's not the problem. If such a feature is not accepted by the guest? How to you fall back? We don't add features which unmake the standard. > That would preserve zero-copy capabilities (as long as you can work > against the shared mem directly, e.g. doing DMA from a physical NIC or > storage device into it) and keep the hypervisor out of the loop. This seems ill thought out. How will you program a NIC via the virtio protocol without a hypervisor? And how will you make it safe? You'll need an IOMMU. But if you have an IOMMU you don't need shared memory. > Is it > too invasive to existing infrastructure or does it have some other pitfalls? You'll have to convince every vendor to implement your addition to the standard. Which is easier than inventing a completely new system, but it's not quite virtio. Cheers, Rusty.
