On 4/13/20 8:47 AM, Alex Williamson wrote: >> You can delay pinning until the device is actually used. That should be >> late enough for the host to figure out whether a paravirtualized IOMMU >> is in place. > So the guest enables the bus master bit in the command register and at > that point we'd stall the VM for an indeterminate length of time while > we potentially pin all memory, and hope that both the user and the host > has the resources to account and allocate that memory, otherwise the > VM suddenly crashes? All of this potentially taking place in the > pre-boot environment to support option ROMs as well. A delay starting > the VM seems a lot more predictable. Thanks, BTW, there are a million ways to speed up VM startup without both complicating the core VM *and* slowing down everybody that gets a speedup from cache-hot pages coming out of the allocator. Use ramfs or hugetlbfs files. Have a bunch of them sitting around, preallocated (and zeroed) and dole them out as VMs start up. Instead of complicating the core VM, do the pre-zeroing in hugetlbfs. Zeroing at the time the pages get added to the pool wouldn't be the worst thing and wouldn't touch the core VM.
