From: Andi Kleen <ak@linux.intel.com>
To: Jim Mattson <jmattson@google.com>
Cc: "Wei Wang" <wei.w.wang@intel.com>,
LKML <linux-kernel@vger.kernel.org>,
"kvm list" <kvm@vger.kernel.org>,
"Paolo Bonzini" <pbonzini@redhat.com>,
"Peter Zijlstra" <peterz@infradead.org>,
"Kan Liang" <kan.liang@intel.com>,
"Ingo Molnar" <mingo@redhat.com>,
"Radim Krčmář" <rkrcmar@redhat.com>,
like.xu@intel.com, "Jann Horn" <jannh@google.com>,
arei.gonglei@huawei.com
Subject: Re: [PATCH v4 04/10] KVM/x86: intel_pmu_lbr_enable
Date: Thu, 3 Jan 2019 09:18:38 -0800 [thread overview]
Message-ID: <20190103171838.GB6118@tassilo.jf.intel.com> (raw)
In-Reply-To: <CALMp9eR0rkF-DuOWyB16q5r8y9DrefsMd1h4GfDkA8nmpWjMEg@mail.gmail.com>
> Yes, but then what happens?
>
> Fast forward to, say, 2021. You're decommissioning all Broadwell
> servers in your data center. You have to migrate the running VMs off
> of those Broadwell systems onto newer hardware. But, with the current
> implementation, the migration cannot happen. So, what do you do? I
> suppose you just never enable the feature in the first place. Right?
There would in theory ways to handle this.
LBRs are normally not required for running correctly, they're not like
instructions, so it would be actually quite reasonable to just return 0
and ignore writes for incompatible machines for them. Your profilers might
return some bogus data, but that is normally acceptable.
In some cases it might be also possible to emulate LBRs of
different types, but I don't think it would be worth the effort.
Yes, but the patches don't do this, so right now you should only enable it when
all systems in your migration pool have compatible LBRs.
-Andi
next prev parent reply other threads:[~2019-01-03 17:18 UTC|newest]
Thread overview: 42+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-12-26 9:25 [PATCH v4 00/10] Guest LBR Enabling Wei Wang
2018-12-26 9:25 ` [PATCH v4 01/10] perf/x86: fix the variable type of the LBR MSRs Wei Wang
2018-12-26 9:25 ` [PATCH v4 02/10] perf/x86: add a function to get the lbr stack Wei Wang
2018-12-26 9:25 ` [PATCH v4 03/10] KVM/x86: KVM_CAP_X86_GUEST_LBR Wei Wang
2018-12-26 9:25 ` [PATCH v4 04/10] KVM/x86: intel_pmu_lbr_enable Wei Wang
2019-01-02 16:33 ` Liang, Kan
2019-01-04 9:58 ` Wei Wang
2019-01-04 15:57 ` Liang, Kan
2019-01-05 10:09 ` Wei Wang
2019-01-07 14:22 ` Liang, Kan
2019-01-08 6:13 ` Wei Wang
2019-01-08 14:08 ` Liang, Kan
2019-01-09 1:54 ` Wei Wang
2019-01-02 23:26 ` Jim Mattson
2019-01-03 7:22 ` Wei Wang
2019-01-03 15:34 ` Jim Mattson
2019-01-03 17:18 ` Andi Kleen [this message]
2019-01-04 10:09 ` Wei Wang
2019-01-04 15:53 ` Jim Mattson
2019-01-05 10:15 ` Wang, Wei W
2018-12-26 9:25 ` [PATCH v4 05/10] KVM/x86: expose MSR_IA32_PERF_CAPABILITIES to the guest Wei Wang
2019-01-02 23:40 ` Jim Mattson
2019-01-03 8:00 ` Wei Wang
2019-01-03 15:25 ` Jim Mattson
2019-01-07 9:15 ` Wei Wang
2019-01-07 18:05 ` Jim Mattson
2019-01-07 18:20 ` Andi Kleen
2019-01-07 18:48 ` Jim Mattson
2019-01-07 20:14 ` Andi Kleen
2019-01-07 21:00 ` Jim Mattson
2019-01-08 7:53 ` Wei Wang
2019-01-08 17:19 ` Jim Mattson
2018-12-26 9:25 ` [PATCH v4 06/10] perf/x86: no counter allocation support Wei Wang
2018-12-26 9:25 ` [PATCH v4 07/10] KVM/x86/vPMU: Add APIs to support host save/restore the guest lbr stack Wei Wang
2018-12-26 9:25 ` [PATCH v4 08/10] perf/x86: save/restore LBR_SELECT on vCPU switching Wei Wang
2018-12-26 9:25 ` [PATCH v4 09/10] perf/x86: function to check lbr user callstack mode Wei Wang
2018-12-26 9:25 ` [PATCH v4 10/10] KVM/x86/lbr: lazy save the guest lbr stack Wei Wang
2018-12-27 20:51 ` Andi Kleen
2018-12-28 3:47 ` Wei Wang
2018-12-28 19:10 ` Andi Kleen
2018-12-27 20:52 ` [PATCH v4 10/10] KVM/x86/lbr: lazy save the guest lbr stack II Andi Kleen
2018-12-29 4:25 ` Wang, Wei W
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