Re: Bug: Windows 2003 fails to install on xen-unstable tip

[George Dunlap <george.dunlap@eu.citrix.com>]

On 23/04/13 16:02, Jan Beulich wrote:
>>>> On 23.04.13 at 16:57, George Dunlap <george.dunlap@eu.citrix.com> wrote:
>> On 23/04/13 15:55, Jan Beulich wrote:
>>>>>> On 23.04.13 at 16:21, George Dunlap <george.dunlap@eu.citrix.com> wrote:
>>>> On 23/04/13 14:00, Jan Beulich wrote:
>>>>>>>> On 23.04.13 at 13:52, George Dunlap <george.dunlap@eu.citrix.com> wrote:
>>>>>> On 23/04/13 12:33, Jan Beulich wrote:
>>>>> Just went through that change again: The only thing that changes
>>>>> is that while rtc_periodic_cb() (simply setting REG_C flags) got
>>>>> called at the end of pt_intr_post(), rtc_periodic_interrupt() now
>>>>> gets called from pt_update_irq(), and therefore takes care of
>>>>> asserting the IRQ itself (which originally happened inside
>>>>> pt_update_irq()) along with setting REG_C flags. Bottom line -
>>>>> all the patch changes is when exactly REG_PF (and possibly
>>>>> REG_IRQF) get set, and whether the IRQ actually gets asserted.
>>>> So that last bit seems to do these things:
>>>> a. Changes when the handling happens in the Xen interrupt handler
>>>> b. Causes assert/deassert to only happen if !(C.PF) && (B.PIE)
>>>>      (Before it happened unconditionally)
>>>> c. Causes C.IRQF=1 only if (same condition above)
>>>>     (Before it happened unconditionally)
>>>> d. Runs destroy_periodic_timer() if C.PF already
>>>>
>>>> So just to figure out what it was that w2k3 wanted, I tried a bunch of
>>>> variations:
>>>>
>>>> * All of above
>>>>     BAD
>>>> * a only; always assert/deassert + set C.IRQF
>>>>     GOOD
>>>> * always assert/deassert, but leave destroy_periodic_timer and IRQF
>>>> setting alone
>>>>     FAIL
>>>> * destroy if C.PF, assert/deassert, set IRQF if setting C.PF (don't
>>>> check B.PIE)
>>>>     FAIL
>>>> * destroy if C.PF, always assert/deassert + set C.IRQF
>>>>     FAIL
>>>> * never destroy, assert/deassert + set C.IRQF if setting C.PF
>>>>     FAIL
>>>> * never destroy, assert/deassert + set C.IRQF if !C.IRQF
>>>>     FAIL
>>>>
>>>> In short, it seems that w2k3 basically expects an unlimited number of
>>>> attempts to actually deliver the interrupt.
>>> Not always doing the deassert/assert pair was actually part of Tim's
>>> subsequent changes - before that it got called conditionally upon
>>> B.PIE, but always set C.IRQF and always deasserted and asserted.
>>> Since we know from the debug log that B.PIE is set, I fail to see how
>>> the code prior to "x86/hvm: Centralize and simplify the RTC IRQ logic"
>>> would have not worked, but the above case turned out GOOD.
>>>
>>> So did his earlier 3 changes perhaps fix the issue, and the fourth re-
>>> introduced it?
>> If you've got a git hash I can try to revert it.
> 527824f41f5fac9cba3d4441b2e73d3118d98837

Reverting that c/s has no effect.

And if I then make the change that made it work before -- namely, 
unconditionally calling rtc_toggle_irq in rtc_periodic_interrupt() -- 
then the w2k3 installer just spins instead of hangs.

At this point I think it's worth asking: is raising needless IRQs and 
running non-used pmtimers really causing that big of an issue? Given 
that I've just spent a whole day trying to debug this, wouldn't it be 
better just to revert all the rtc-related changesets from 620d5da?

  -George