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Subject: Re: [virtio-comment] RFC: virtio-hostmem (+ Continuation of discussion from [virtio-dev] Memory sharing device)
* Frank Yang (lfy@google.com) wrote: > virtio-hostmem is a proposed way to share host memory to the guest and > communicate notifications. One potential use case is to have userspace > drivers for virtual machines. > > The latest version of the spec proposal can be found at > > https://github.com/741g/virtio-spec/blob/master/virtio-hostmem.tex > > The revision history so far: > > https://github.com/741g/virtio-spec/commit/7c479f79ef6236a064471c5b1b8bc125c887b948 > - originally called virtio-user > https://github.com/741g/virtio-spec/commit/206b9386d76f2ce18000dfc2b218375e423ac8e0 > - renamed to virtio-hostmem and removed dependence on host callbacks > https://github.com/741g/virtio-spec/commit/e3e5539b08cfbaab22bf644fd4e50c00ec428928 > - removed a straggling mention of a host callback > https://github.com/741g/virtio-spec/commit/61c500d5585552658a7c98ef788a625ffe1e201c > - Added an example usage of virtio-hostmem > > This first RFC email includes replies to comments from mst@redhat.com: > > > \item Guest allocates into the PCI region via config virtqueue messages. > > Michael: OK so who allocates memory out of the PCI region? > Response: > > Allocation will be split by guest address space versus host address space. > > Guest address space: The guest driver determines the offset into the BAR in > which to allocate the new region. The implementation of the allocator > itself may live on the host (while guest triggers such allocations via the > config virtqueue messages), but the ownership of region offsets and sizes > will be in the guest. This allows for the easy use of existing guest > ref-counting mechanisms such as last close() calling release() to clean up > the memory regions in the guest. > > Host address space: The backing of such memory regions is considered > completely optional. The host may service a guest region with a memory of > its choice that depends on the usage of the device. The time this servicing > happens may be any time after the guest communicates the message to create > a memory region, but before the guest destroys the memory region. In the > meantime, some examples of how the host may respond to the allocation > request: > > - The host does not back the region at all and a page fault happens. Note that a mapping missing on the host wont necessarily turn into a page fault in the guest; on qemu for example, if you have a memory region like this where the guest accesses an area with no mapping, I think we hit a kvm error. Dave -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
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