Re: [Regression, bisected] a3e06bbe8445f57eb949e6474c5a9b30f24d2057:KVM: emulate lapic tsc deadline timer for guest"

[Alexey Zaytsev]

On Tue, Dec 20, 2011 at 22:19, Liu, Jinsong <jinsong.liu@xxxxxxxxx> wrote:
> Alexey Zaytsev wrote:
>> On Tue, Dec 20, 2011 at 21:21, Liu, Jinsong <jinsong.liu@xxxxxxxxx>
>> wrote:
>>> Linus Torvalds wrote:
>>>> On Tue, Dec 20, 2011 at 1:51 AM, Alexey Zaytsev
>>>> <alexey.zaytsev@xxxxxxxxx> 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 Âprepared 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.
>>>>
>>>> Â Â Â Â Â Â Â Â Â ÂLinus
>>>
>>> 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


>
> Thanks,
> Jinsong
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/