Re: [PATCH] KVM: x86: Don't deliver PIC interrupts to disabled APICs - v2

["Yang, Sheng" <sheng.yang@intel.com>]

On Thursday 23 October 2008 04:44:48 Jan Kiszka wrote:
> Jan Kiszka wrote:
> > Jan Kiszka wrote:
> >> Jan Kiszka wrote:
> >>> Avi Kivity wrote:
> >>>> Jan Kiszka wrote:
> >>>>> [ taking Sheng's comments into account ]
> >>>>>
> >>>>> The logic of kvm_apic_accept_pic_intr has a minor, practically hardly
> >>>>> relevant incorrectness: PIC interrupts are still delivered even if
> >>>>> the APIC of VPU0 (BSP) is disabled. This does not comply with the
> >>>>> Virtual Wire mode according to the Intel MP spec.
> >>>>
> >>>> This breaks Windows XP with the Standard PC HAL, so I am unapplying
> >>>> this patch.
> >>>
> >>> Hmm, this points to either an APIC setup or BIOS bug. To my
> >>> understanding, the Standard PC HAL should not fiddle with the APIC, so
> >>> what the BIOS leaves behind should counts. But I think I found no
> >>> traces of APIC manipulation in rombios32.c.
> >>
> >> Manipulation on UP systems. There is fiddling for SMP. But I will check
> >> again.
> >
> > I take everything back: For yet unknown reasons Windows' standard HAL
> > actually decides to disable the APIC actively. Either there is a
> > short-path around a disabled APIC for Virtual Wire mode in Real Live
> > (though I fail to read this out of the spec), or Windows simply has a
> > bug here (MS insists on NOT supporting the Standard HAL on APIC systems
> > [1] - precisely the setup KVM is providing). Sheng, any comments on
> > this? Guess we have to live with the previous version, maybe with some
> > refactoring + commenting.
>
> I was curious and played with my corresponding qemu patch [1]: It works
> with the same Windows image that hangs under KVM. Then I looked at a
> prominent guest-visible difference: the reported CPU type. QEMU claims
> to provide a CPU called "qemu64" by default. Changing this to Pentium2
> or newer makes Windows issue the lethal APIC disable command. On the
> other hand, calling kvm with "-cpu pentium" makes Windows boot again.
>
> However, I still can't tell from this if we see a Windows bug or if the
> change is incorrect (but me feeling tends to the former).

Confirmed that "-cpu pentium" solve the problem. But...

Sorry for that I've found that I neglected some info on the spec. SDM 3B 
5.3.1 "External Interrupts".

"When the local APIC is global/hardware disabled, these pins are configured as 
INTR and NMI pins, respectively."

So, it's right to inject PIC interrupt when LAPIC is hardware disabled. I 
think we can drop this patch...

(I will be more careful on such kind of issues next time...)
--
regards
Yang, Sheng

>
> Jan
>
> [1] http://permalink.gmane.org/gmane.comp.emulators.qemu/31429