Re: [Xen-devel] [Patch] x86/HVM: Fix RTC interrupt modelling

[Tim Deegan <tim@xxxxxxx>]

At 11:17 +0000 on 10 Feb (1392027468), Andrew Cooper wrote:
> The current code has a pathological case, tickled by the access pattern of
> Windows 2003 Server SP2.  Occasonally on boot (which I presume is during a
> time calibration against the RTC Periodic Timer), Windows gets stuck in an
> infinite loop reading RTC REG_C.  This affects 32 and 64 bit guests.
> 
> In the pathological case, the VM state looks like this:
>   * RTC: 64Hz period, periodic interrupts enabled
>   * RTC_IRQ in IOAPIC as vector 0xd1, edge triggered, not pending
>   * vector 0xd1 set in LAPIC IRR and ISR, TPR at 0xd0
>   * Reads from REG_C return 'RTC_PF | RTC_IRQF'
> 
> With an intstrumented Xen, dumping the periodic timers with a guest in this
> state shows a single timer with pt->irq_issued=1 and pt->pending_intr_nr=2.
> 
> Windows is presumably waiting for reads of REG_C to drop to 0

s/presumably/definitely/; we disassembled the kernel code in question.

>, and reading
> REG_C clears the value each time in the emulated RTC.  However:
> 
>   * {svm,vmx}_intr_assist() calls pt_update_irq() unconditionally.
>   * pt_update_irq() always finds the RTC as earliest_pt.

This is, AFAICT, because we are in no-missed-ticks mode, and there are
multiple RTC ticks queued waiting to be delivered.

Cheers,

Tim.

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel