Re: [Xen-devel] [RFC] ARM VM System Sepcification

[Michael Casadevall <michael.casadevall@xxxxxxxxxx>]

On 03/06/2014 05:46 PM, Paolo Bonzini wrote:
> Il 06/03/2014 09:52, Robie Basak ha scritto:
> > On Sat, Mar 01, 2014 at 03:27:56PM +0000, Grant Likely wrote:
> > > I would also reference section 3.3 (Boot Option Variables Default Boot
> > > Behavior) and 3.4.1.1 (Removable Media Boot Behavior) here. It's 
> > > fine to
> > > restate the meaning of the requirement in this spec, but the UEFI spec
> > > is the authoritative source. Distributed VM disk images fall under the
> > > same scenario as the firmware not having any valid boot variables.
> > What happens when the VM is first booted without boot variables, but
> > then the OS expects to be able to set boot variables and see them on
> > next boot?
> UEFI scans the devices; looks for an EFI system partition on the 
> disks; and builds a default boot order.
> > If possible, I would prefer to mandate that the host implementation is
> > permitted to no-op (or otherwise disable) boot variable write operations
> > altogether to avoid having to deal with this. In the common case, I
> > don't see why an OS installation shipped via a VM disk image would need
> > to write boot variables anyway.
> >
> > Would there be any adverse consequences to doing this?
> Given the experience on x86 UEFI, no.
>
> Unlike bare metal, it is common to run UEFI VMs without persistent 
> flash storage.  In this case the boot variables and boot order are 
> rebuilt on the fly on every boot, and it just works for both Windows 
> and Linux; there's no reason why it should be any different for ARM.
While I realize in the real world, we can live with non-persistent boot 
variables, this is a *direct* violation of the UEFI spec; we can't call 
our VMs UEFI-compatible if we do this.
However, I've been looking at the spec, and I think we're within spec if 
we save the variables on the HDD itself. There's some support for this 
already (Firmware Block Volume Device), but its possible we could 
implement boot variables as a file on system partition (UEFI's default 
search order can be used to figure out which variable file to use, or 
some sorta fingerprinting system). The biggest trick though is that 
UEFI's Runtime Services will need to be able to write this file, which 
may require us move a large chunk of UEFI to runtime services so the FAT 
filesystem stuff can stick around. If we give it a proper partition, 
then we can just do raw block read/writes. This however would require us 
mandating that said partition exists, and making sure there aren't any 
hidden gotchas in invoking this.
Obviously this isn't ideal, but this might be the middle road solution 
we need here. I can dig through Tiano to get a realistic idea of how 
hard this will be in reality if we want to seriously look at this option.
> > My reason is that this would save us from blocking a general OpenStack
> > implementation on ARM by requiring that these pieces are implemented
> > further up the stack first, when it would bring actual gain to doing so.
> >
> > This would not preclude host implementations from implementing writeable
> > variables, or guests from using them. Just that for a _portable VM disk
> > image_, the OS on it cannot assume that this functionality is present.
> This is already the case for most OSes.  Otherwise you wouldn't be 
> able to move a hard disk from a (physical) machine to another.
> I strongly suggest that you take a look at the work done in Tiano 
> Core's OvmfPkg, which has support for almost every QEMU feature thanks 
> to the work of Laszlo Ersek and Jordan Justen.
> In particular, OvmfPkg has support for specifying a boot order in the 
> VM configuration (which maps to the "-boot" option in QEMU). In this 
> case, the UEFI boot order is overridden by a variable that is placed 
> in some architecture-specific firmware configuration mechanism (on x86 
> we have one called fw_cfg, on ARM you could look at the fdt).  This 
> predates UEFI and is not a UEFI variable; in fact is is a list of 
> OpenFirmware device paths.  UEFI will match the OF paths to UEFI 
> paths, and use the result to build a UEFI boot order.
This lines up with work to make Tiano itself run on FDT to handle 
varying boot configurations. Is this behaviour and the DT nodes codified 
anywhere?
> Paolo

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