On 25/01/2017 17:43, Alex Williamson wrote: > On Wed, 25 Jan 2017 12:32:21 +0800 > Peter Xu <pet...@redhat.com> wrote: >> I have similar question as well above - IIUC the RCU read lock >> protects us from not losing the references of memory objects, however >> in our case even after we release the lock, we are still using the >> backend ram (vaddr) since we have set it up inside kernel to build up >> the IO page table. After that, the kernel/device should be able to >> write to addresses of that backend ram any time.
I don't think that's what happens. As far as I understand, VFIO pins the pages corresponding to vaddr, not vaddr itself. The memory backend is mmap-ed memory; when you hot-unplug it the munmap releases the VMA and loses the connection between QEMU's virtual address space and the pages. However, the pages stay pinned and stay mapped into VFIO's own IOMMU page tables. So if a guest does a memory hot-unplug without IOMMU unmap, it would keep on seeing the content of the hot-unplugged memory, and the host could not release the pages, but the guest cannot overwrite QEMU data structures. Paolo
