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SubjectRe: [Regression, bisected] a3e06bbe8445f57eb949e6474c5a9b30f24d2057: KVM: emulate lapic tsc deadline timer for guest"
On Tue, Dec 20, 2011 at 22:26, Liu, Jinsong <jinsong.liu@intel.com> wrote:
> Alexey Zaytsev wrote:
>> On Tue, Dec 20, 2011 at 22:19, Liu, Jinsong <jinsong.liu@intel.com>
>> wrote:
>>> Alexey Zaytsev wrote:
>>>> On Tue, Dec 20, 2011 at 21:21, Liu, Jinsong <jinsong.liu@intel.com>
>>>> wrote:
>>>>> Linus Torvalds wrote:
>>>>>> On Tue, Dec 20, 2011 at 1:51 AM, Alexey Zaytsev
>>>>>> <alexey.zaytsev@gmail.com> wrote:
>>>>>>>
>>>>>>> Let me clarify the situation.
>>>>>>>
>>>>>>> Before this commit, the tsc was advertised in cpuid, and it was
>>>>>>> handled, if I understand things correctly, by qemu.
>>>>>>> After this commit, the tsc is advertised in cpuid, and is handled
>>>>>>> in the kernel, but only after qemu issues KVM_CREATE_IRQCHIP. If
>>>>>>> it does not issue the ioctl, the kernel just discards any wrmsrs
>>>>>>> done to the tsc. This does not look like an Illumos problem to
>>>>>>> me. Linux guests kind of work here, because they are  prepared
>>>>>>> to work on utterly broken hardware. Good for you, but please
>>>>>>> don't break less-prepared guests.
>>>>>>
>>>>>> Yes. This is a regression, and needs to be fixed.
>>>>>>
>>>>>> Liu, if you don't have time to debug it, we'll just revert the
>>>>>> commit. It's that easy. Regressions are not allowed. There are no
>>>>>> excuses.
>>>>>>
>>>>>> In particular, saying "just wait for qemu-kvm" is not an
>>>>>> acceptable answer, because the point is that things *used* to
>>>>>> work, and they broke. No "change it to work with the new kernel"
>>>>>> allowed, except for some *very* rare critical circumstances
>>>>>> (usually "major security-bug that we had to fix, and people who
>>>>>> relied on it are thus out of luck").
>>>>>>
>>>>>> Commit a3e06bbe8445 still seems to revert cleanly, so that is the
>>>>>> easy option.
>>>>>>
>>>>>> That said, it sounds like maybe another solution is to start with
>>>>>> the TSC_DEADLINE timer bit in cpuid cleared, and only setting it
>>>>>> after the KVM_CREATE_IRQCHIP ioctl.
>>>>>>
>>>>>> In fact, the patch is clearly buggy, in that it apparently doesn't
>>>>>> emulate TSC_DEADLINE correctly and natively on its own.
>>>>>>
>>>>>> Jan, Marcelo, Avi - is there a quick fix, or should I just revert?
>>>>>>
>>>>>> And please don't *EVER* tell people that they should just work
>>>>>> around regressions. Regressions are absolutely unacceptable.
>>>>>> Kernel people need to understand that.
>>>>>>
>>>>>>                    Linus
>>>>>
>>>>> Yes, my fault to say 'walk around' before knowing Alex's issue
>>>>> clearly.
>>>>>
>>>>> After Alex send his last email to clarify the situation, I have
>>>>> checked the bug. Basically it caused from
>>>>> 1. qemu didn't issue KVM_CREATE_IRQCHIP, hence
>>>>> irqchip_in_kernel(kvm) fail when setup vcpu lapic logic at
>>>>> kvm_arch_vcpu_init();
>>>>> 2. tsc deadline work based on vcpu lapic, hence break illumos;
>>>>>
>>>>> A fix is to update cpuid, as you said, setting it after
>>>>> KVM_CREATE_IRQCHIP. I just wonder is there any better solution? so
>>>>> I ask Alex his environment to setup at my side to do more test. If
>>>>> you think kvm tsc deadline timer patch itself not clean, please
>>>>> tell me.
>>>>
>>>> [Removed Linus from CC]
>>>>
>>>> If your internet connection is good enough, I could just pass you
>>>> the disk image. It should be around 5g when compressed. Otherwise,
>>>> you can download an openindiana disk image from
>>>> http://openindiana.org. You probably want the text-only "server"
>>>> one. The TSC code has changed since that release, but the old one
>>>> does not work as well.
>>>
>>> I mean I will build environment at my side with your qemu version
>>> (where do you pull from? commit number?) --> just to verify whether
>>> we can solve it at qemu side, or, solve it by cpuid at kvm side. I
>>> will present a patch to fix it and you can test at your environment.
>>>
>>>>
>>>> Do I get it right that Linux does not use the tsc deadline timer?
>>>> I've added printks to the
>>>> kvm_get_lapic_tscdeadline_msr/kvm_set_lapic_tscdeadline_msr
>>>> functions, and I swear I've seen them trigger a few times with the
>>>> Linux guest. But I can't find the code that is issuing the msr
>>>> reads/writes in the Linux kernel.
>>>
>>> I will check it tomorrow, too late now.
>>> and would you please tell me, does illumos use tsc deadline timer? I
>>> lack this basic information.
>>
>> It does, if available. Please check
>> https://github.com/illumos/illumos-gate/blob/master/usr/src/uts/i86pc/io/pcplusmp/apic_timer.c#L290
>>
>
> and where do you pull qemu? and commit number? I usually use Avi's qemu-kvm.

From git://git.qemu.org/qemu.git, commit
62ba9f3662c5481f31160daf25474c8f22d6b3d2
Qemu 0.15.92 exhibited the same behavior.


>
> Thanks,
> Jinsong
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