[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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

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 (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?


Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.