Re: [Xen-devel] [RFC v2] ARM VM System Specification

[Christoffer Dall <christoffer.dall@xxxxxxxxxx>]

On Tue, Jun 10, 2014 at 09:18:34PM +0200, Paolo Bonzini wrote:
> Il 10/06/2014 20:56, Paolo Bonzini ha scritto:
> >Il 10/06/2014 20:08, Peter Maydell ha scritto:
> >>On 10 June 2014 18:04, Christopher Covington <cov@xxxxxxxxxxxxxx> wrote:
> >>>On 06/10/2014 10:42 AM, Peter Maydell wrote:
> >>>>I just noticed that this doesn't mandate that the platform
> >>>>provides an RTC. As I understand it, the UEFI spec mandates
> >>>>that there's an RTC (could somebody more familiar with UEFI
> >>>>than me confirm/deny that?) so we should probably put one here.
> >>>
> >>>Pardon my ignorance, but what exactly disqualifies Generic Timer
> >>>implementations from being used as Real Time Clocks?
> >>
> >>So my naive view was that an RTC actually had to have
> >>support for dealing with real (wall) clock time, ie
> >>knowing it's 2014 and not 1970. The generic timers are
> >>just timers. Or am I wrong and UEFI doesn't really
> >>require that?
> >
> >The real-time clock provides four UEFI runtime services (GetTime,
> >SetTime, GetWakeupTime, SetWakeupTime).  The spec says that you can
> >return EFI_DEVICE_ERROR from GetTime/SetTime if "the time could not be
> >retrieved/set due to a hardware error", but I don't think this is enough
> >to make these two optional.  By comparison, GetWakeupTime/SetWakeupTime
> >can also return EFI_UNSUPPORTED.
> >
> >So I agree that the RTC is required in UEFI.
> 
> ... that said, just like I thought was the case for the serial
> console, do we need to specify the exact hardware models?
> 
> We can just say that the VM can expect UEFI boot and runtime
> services to work.  This includes variable services, time services,
> the serial console protocols and more.  It's up to the
> implementation to provide enough devices to support the firmware,
> and it's out of this spec's scope to specify the firmware's
> implementation.
> 
> I think even the serial devices should be removed.
> 
The problem is that the most common user problem with ARM VMs are that
they boot the thing, and then get no output.  So we wanted some way to
make sure we know that the kernel should be able to print to a console.

UEFI does provide DBG2 output, but that's only during boot time service
(so I'm told), and we need to mandate something that will work  when
the kernel boots.

If kernels actually do use the UEFI runtime services and have no need
for direct access to an RTC when runing in a UEFI compliant system, then
I agree with not specifying the hardware details.

-Christoffer

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