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Date: Wed, 21 Dec 2011 04:44:41 +0800 From: "Liu, Jinsong" <jinsong.liu@...el.com> To: Alexey Zaytsev <alexey.zaytsev@...il.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> Subject: RE: [Regression, bisected] a3e06bbe8445f57eb949e6474c5a9b30f24d2057: KVM: emulate lapic tsc deadline timer for guest" Alexey Zaytsev wrote: > On Tue, Dec 20, 2011 at 22:26, Liu, Jinsong <jinsong.liu@...el.com> > wrote: >> Alexey Zaytsev wrote: >>> On Tue, Dec 20, 2011 at 22:19, Liu, Jinsong <jinsong.liu@...el.com> >>> wrote: >>>> Alexey Zaytsev wrote: >>>>> On Tue, Dec 20, 2011 at 21:21, Liu, Jinsong >>>>> <jinsong.liu@...el.com> wrote: >>>>>> Linus Torvalds wrote: >>>>>>> On Tue, Dec 20, 2011 at 1:51 AM, Alexey Zaytsev >>>>>>> <alexey.zaytsev@...il.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. > OK, thanks. I will fix it tomorrow. Thanks, Jinsong -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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