Re: [PATCH RFCv2 2/4] i386/pc: relocate 4g start to 1T where applicable

[Dr. David Alan Gilbert]

* Daniel P. Berrangé (berrange@redhat.com) wrote:
> 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.

And the upstream code is now pretty much identical except for the
default;  note that for TCG you do need to keep to 40 I think.

Dave
> 
> >  * 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
> -- 
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> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK