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Re: [Xen-devel] [PATCH] Provide HVM guest RTC change notification


What's the status of this discussion/patch ? It would be nice to
get some closure on this. I know everyone's busy.

My understanding of the current concerns from Christian are (and
these are my interpretation of his comments):

- simply use a inequality test in time offsets to trigger setting
  the new time offset.

  My rationale for the as-submitted patch, was that there is an
  existing per-second mechanism that tries to emulate the set and
  uip bits and timing.  Time changes appear to happen during this
  per-second mechanism.  While it looks simple to just modify the
  time if the offset is detected to have changed, it would appear
  that this would negatively impact readers of the RTC and
  circumvent the work done by the per-second mechanism.  More
  simply, it wasn't clear to me that modifying the time wouldn't
  break guests unless the per-second mechanism was utilized.

- don't use the VIRQ mechanism for time-offset (RTC write)
  notifications.  Use an ioreq to qemu.

  The as-submitted patch used the VIRQ mechanism for a few
  reasons.  By building upon the domain event mechanism/virq,
  this low frequency event is handled in a fashion quite
  like other low-frequency domain events such as shutdown and
  crash. I was looking for a low-cost, low-impact, fire-and-forget
  means to signal an RTC change.

  The ioreq approach is interesting.  I'm uncertain about it for
  the following reasons, which some small additional explanation
  would help me with.  Adding an ioreq for time change seems quite
  out of character with the other ioreq events.  Additionally,
  qemu processing (and writing a store entry) are likely to be
  rather expensive.  Wouldn't this also cause undue delay in the
  guest awaiting the completion of the ioreq ?

It is quite possible that I have misunderstood Christian's comments.
If so, I'd love to better understand the issues. I really don't care
what the final implementation is, other than the usual expectations
of robustness, reasonable performance and clarity. Perhaps with a
bit more information, this can be cleared up and closed up.  I'd like
to get back to the capabilities that existed before the RTC code
was moved into the hypervisor.

Thanks !

Christian Limpach wrote:
On 3/20/07, Ben Thomas <bthomas@xxxxxxxxxxxxxxx> wrote:
This patch restores the capabilities initially provided in
changeset 10010.  When the RTC code was moved into the hypervisor
(a good move), the control plane lost the ability to read the
current time offset as well as to receive notification of changes
to the RTC time base by a guest. This patch reintroduces these.

Additionally, there is a small window at initialization time in
which the time offset may be set but not noticed or used. If
xc_domain_set_time_offset is called before the domain is started,
the offset won't be noticed until the next second update. An HVM
guest could read the RTC in this window and get an unintended
result. This patch closes the window.

Isn't the "s->time_offset_seconds !=
s->pt.vcpu->domain->time_offset_seconds" condition an appropriate test
whether you need to call rtc_copy_date?

The patch builds upon the existing change notification mechanism
provided by VIRQ_DOM_EXC. I stopped short of renaming the
releaseDomain watch to something like domainEvent as it wasn't
clear who might be relying upon the existing name. A patch for
that would be easy to create, though.

The domain exception virq is already quite overloaded as is and it
causes xend to do a quite expensive scan of all the domain

I think using an ioreq to qemu would be a more appropriate mechanism
to signal that the guest has changed the timeoffset.  The ioreq would
include the delta by which the offset was changed.  qemu could then
update the timeoffset stored in xenstore, since we would want to make
the changed timeoffset persist across reboots.


Last, the code introduced into qemu by 10010 is no longer really
connected to anything and could be removed. It's found in
tools/ioemu/patches/domain-timeoffset and can probably be excised
at some suitable point. Currently, it's relatively harmless.

Signed-off-by: Ben Thomas (ben@xxxxxxxxxxxxxxx)

Ben Thomas                                         Virtual Iron Software
bthomas@xxxxxxxxxxxxxxx                            Tower 1, Floor 2
978-849-1214                                       900 Chelmsford Street
                                                   Lowell, MA 01851

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