On Tue, Feb 15, 2022 at 10:53:58AM +0100, Gerd Hoffmann wrote: > Hi, > > > I don't know what behavior should be if firmware tries to program > > PCI64 hole beyond supported phys-bits. > > Well, you are basically f*cked. > > Unfortunately there is no reliable way to figure what phys-bits actually > is. Because of that the firmware (both seabios and edk2) tries to place > the pci64 hole as low as possible. > > The long version: > > qemu advertises phys-bits=40 to the guest by default. Probably because > this is what the first amd opteron processors had, assuming that it > would be a safe default. Then intel came, releasing processors with > phys-bits=36, even recent (desktop-class) hardware has phys-bits=39. > Boom. > > End result is that edk2 uses a 32G pci64 window by default, which is > placed at the first 32G border beyond normal ram. So for virtual > machines with up to ~ 30G ram (including reservations for memory > hotplug) the pci64 hole covers 32G -> 64G in guest physical address > space, which is low enough that it works on hardware with phys-bits=36. > > If your VM has more than 32G of memory the pci64 hole will move and > phys-bits=36 isn't enough any more, but given that you probably only do > that on more beefy hosts which can take >= 64G of RAM and have a larger > physical address space this heuristic works good enough in practice. > > Changing phys-bits behavior has been discussed on and off since years. > It's tricky to change for live migration compatibility reasons. > > We got the host-phys-bits and host-phys-bits-limit properties, which > solve some of the phys-bits problems. > > * host-phys-bits=on makes sure the phys-bits advertised to the guest > actually works. It's off by default though for backward > compatibility reasons (except microvm). Also because turning it on > breaks live migration of machines between hosts with different > phys-bits.
RHEL has shipped with host-phys-bits=on in its machine types sinec RHEL-7. If it is good enough for RHEL machine types for 8 years, IMHO, it is a sign that its reasonable to do the same with upstream for new machine types. > * host-phys-bits-limit can be used to tweak phys-bits to > be lower than what the host supports. Which can be used for > live migration compatibility, i.e. if you have a pool of machines > where some have 36 and some 39 you can limit phys-bits to 36 so > live migration from 39 hosts to 36 hosts works. RHEL machine types have set this to host-phys-bits-limit=48 since RHEL-8 days, to avoid accidentally enabling 5-level paging in guests without explicit user opt-in. > What is missing: > > * Some way for the firmware to get a phys-bits value it can actually > use. One possible way would be to have a paravirtual bit somewhere > telling whenever host-phys-bits is enabled or not. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
