Thanks, but I don't understand why nobody mentions actually WHY it is being locked. I eventually found the answer by listening to Alex Williamson's talk on the subject, where he mentions that IOMMU chips lack "page faulting" machanism. He also suggests that it may be possible that in the future it will be added to x86 PCs, so I think that it is important that we talk about it rather than just take this memory pinning situation for granted.
Thanks for mentioning memory hotplugging - I've looked into
virtio-mem project, which seems to address this, but as you
mentioned (and thank you for stating it clearly as it took me a
long time to figure it out) it requires manual interventions from
the admin.
Best wishes,
Daniel
On Fri, Sep 15, 2023 at 02:41:50AM +0100, Daniel Krajnik wrote:Hi I'm trying to Passthrough a GPU to a VM and have it still share its free memory with the host (just like it would with virtio-balloon). Is this something virtio-mem is capable of? I've tried to run it, but it immediately took over 9GB of memory: virt-install --name test --cpu cell0.cpus=0,cell0.memory=4194304,cell1.cpus=1,cell1.memory=4194304 --memory maxMemory=65536,maxMemory.slots=8 --memdev model=virtio-mem,target.node=0,target.block=2048,target.size=1000 --disk size=400 --network network=ai --location=/tmp/debian-12.1.0-amd64-netinst.iso --os-variant=debian11 --graphics none --extra-args "console=ttyS0When using PCI device assignment, the VFIO driver requires that libvit pin guest memory into RAM https://gitlab.com/libvirt/libvirt/-/blob/19484ccac5cb1586f9d10d3e6eb9b93ee82885c6/src/qemu/qemu_domain.c#L9567 so that's why you'll see the immediate memory usage. I dont' think there's any way to workaround this limitation automatically. Closest you can get is to actually hot-unplug the RAM your guest isn't using, but that needs manual work to re-hot-plug it if the guest later needs more RAM, so that's not very friendly. With regards, Daniel
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