Messages in this thread | | | From | Alexey Zaytsev <> | Date | Tue, 20 Dec 2011 22:22:31 +0200 | Subject | Re: [Regression, bisected] a3e06bbe8445f57eb949e6474c5a9b30f24d2057: KVM: emulate lapic tsc deadline timer for guest" |
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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
> > Thanks, > Jinsong -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.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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