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

[Grant Likely <grant.likely@xxxxxxxxxx>]

On Wed, Jun 11, 2014 at 12:58 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> On Wednesday 11 June 2014 12:33:30 Grant Likely wrote:
>> On Tue, Jun 10, 2014 at 7:56 PM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>> > 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.
>>
>> In practical terms, yes the VM needs to provide an RTC interface, but
>> I don't think it needs to appear in this spec, even if the kernel
>> accesses it directly. Portable images should use the UEFI service.
>
> It sounds like it should be in the spec then, if we want people building
> portable images to include the efi-rtc driver.

Fair enough.

g.

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