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

["Daniel P. Berrangé" <berrange@redhat.com>]

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
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