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

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Message-ID: <CAB9v_DEA4HJ2Xyc_fMrZ9gi9oNmKPTmG5LLCiwgsPDHbhgRaQw@mail.gmail.com>
Date:	Tue, 20 Dec 2011 11:51:17 +0200
From:	Alexey Zaytsev <alexey.zaytsev@...il.com>
To:	"Liu, Jinsong" <jinsong.liu@...el.com>
Cc:	Kernel development list <linux-kernel@...r.kernel.org>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Avi Kivity <avi@...hat.com>,
	"Garrett D'Amore" <garrett@...enta.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [Regression, bisected] a3e06bbe8445f57eb949e6474c5a9b30f24d2057:
 KVM: emulate lapic tsc deadline timer for guest"

On Tue, Dec 20, 2011 at 11:26, Liu, Jinsong <jinsong.liu@...el.com> wrote:
> Alexey Zaytsev wrote:
>> On Tue, Dec 20, 2011 at 10:53, Liu, Jinsong <jinsong.liu@...el.com>
>> wrote:
>>> Alexey Zaytsev wrote:
>>>> On Wed, Dec 14, 2011 at 11:37, Alexey Zaytsev
>>>> <alexey.zaytsev@...enta.com> wrote:
>>>>> On Mon, Dec 12, 2011 at 10:13, Liu, Jinsong <jinsong.liu@...el.com>
>>>>> wrote:
>>>>>> Alexey Zaytsev wrote:
>>>>>>> Hi.
>>>>>>>
>>>>>>> After a recent change, qemu --enable-kvm (both 0.15.92 and 1.0)
>>>>>>> fails to boot illumos. The OS gets stuck pretty late in the boot
>>>>>>> process, without any hints from the guest. MDB (the illumos
>>>>>>> kenrnel debugger) failed to clarify the situation at once, the
>>>>>>> kernel seems to be stuck idling. I've bisected the problem to
>>>>>>> commit a3e06bbe8445f57eb949e6474c5a9b30f24d2057. Please let me
>>>>>>> know if you need me to collect any debug information or test any
>>>>>>> patches.
>>>>>>
>>>>>> Alexey,
>>>>>>
>>>>>> Does illumos use tsc deadline timer? and do you run it at Intel
>>>>>> platform?
>>>>>>
>>>>>> If yes, would you please help me to collect debug information by
>>>>>> add some printk points at kvm tsc deadline timer logic? currently
>>>>>> I totally have no clue to figure out what the issue root from :)
>>>>>> and, would you please check illumos tsc deadline timer logic by
>>>>>> confirm whether illumos 1). Enumerate tsc deadline timer
>>>>>> capability by CPUID; 2). Enable tsc deadline timer mode by lapic
>>>>>> MMIO; 3). Start tsc deadline timer by WRMSR;
>>>>>
>>>>> I think, here's what's going on. We get into
>>>>> kvm_get_lapic_tscdeadline_msr/kvm_set_lapic_tscdeadline_msr, but
>>>>> qemu does not use the KVM_CREATE_IRQCHIP ioctl, so we are running
>>>>> without vcpu->arch.apic and the functions do nothing. And the
>>>>> Linux guest just handles broken tsc a lot  better, so it survives.
>>>>> Both guests seem to work fine with qemu-kvm, which uses the ioctl.
>>>>>
>>>>
>>>> Hi. Do you need any more info on thins?
>>>
>>> Seems illumos works fine w/ qemu-kvm, that's enough. Maybe you need
>>> to wait qemu-kvm sync into qemu?
>>>
>>
>> Qemu-kvm is fine for me, but this is a regression and should be fixed.
>
> Sorry, I haven't your environment to debug illumos regression.
> I don't think it directly related to kvm tsc deadline timer, and I think it's no value to do it now, since
> for linux guest it works fine w/ both qemu & qemu-kvm,
> for illumos guest you just need wait qemu-kvm sync into qemu.
>

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.

You can just run a Linux guest in qemu, and see what happens in
kvm_get_lapic_tscdeadline_msr and kvm_set_lapic_tscdeadline_msr.
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