Re: [PATCH v6 00/16] KVM: x86/pmu: Add *basic* support to enable guest PEBS via DS

From: Like Xu
Date: Thu May 20 2021 - 21:37:54 EST

On 2021/5/19 9:44, Liuxiangdong wrote:

On 2021/5/18 20:40, Xu, Like wrote:
On 2021/5/18 20:23, Liuxiangdong wrote:

On 2021/5/17 14:38, Like Xu wrote:
Hi xiangdong,

On 2021/5/15 18:30, Liuxiangdong wrote:

On 2021/5/11 10:41, Like Xu wrote:
A new kernel cycle has begun, and this version looks promising.

The guest Precise Event Based Sampling (PEBS) feature can provide
an architectural state of the instruction executed after the guest
instruction that exactly caused the event. It needs new hardware
facility only available on Intel Ice Lake Server platforms. This
patch set enables the basic PEBS feature for KVM guests on ICX.

We can use PEBS feature on the Linux guest like native:

   # perf record -e instructions:ppp ./br_instr a
   # perf record -c 100000 -e instructions:pp ./br_instr a

Hi, Like.
Has the qemu patch been modified? ?

I think the qemu part still works based on
609d7596524ab204ccd71ef42c9eee4c7c338ea4 (tag: v6.0.0).

Yes. I applied these two qemu patches to qemu v6.0.0 and this kvm patches set to latest kvm tree.

I can see pebs flags in Guest(linux 5.11) on the IceLake( Model: 106 Model name: Intel(R) Xeon(R) Platinum 8378A CPU),
and i can use PEBS like this.

    #perf record -e instructions:pp

It can work normally.

But  there is no sampling when i use "perf record -e events:pp" or just "perf record" in guest
unless i delete patch 09 and patch 13 from this kvm patches set.

With patch 9 and 13, does the basic counter sampling still work ?
You may retry w/ "echo 0 > /proc/sys/kernel/watchdog" on the host and guest.

In fact, I didn't use "echo 0 > /proc/sys/kernel/watchdog" when I tried PEBS patches V3 on Icelake.
Why should we use it now?  What does it have to do with sampling?

In the recent patch sets, we disable the guest PEBS when the guest
PEBS counter is cross mapped to a host PEBS counter with a
different index.

When we use the watchdog feature on the Intel platforms,
it may takes a cycle hw counter on the host and it may cause
the guest PEBS counter temporarily disabled if it's cross mapped.

Check patch 0013 for more details.


Have you tried "perf record -e events:pp" in this patches set? Does it work normally?

All my PEBS testcases passed. You may dump guest msr traces from your testcase with me.

Xiangdong Liu

When the LBR qemu patch receives the ACK from the maintainer,
I will submit PBES qemu support because their changes are very similar.

Please help review this version and
feel free to add your comments or "Reviewed-by".

Like Xu

To emulate guest PEBS facility for the above perf usages,
we need to implement 2 code paths:

1) Fast path

This is when the host assigned physical PMC has an identical index as
the virtual PMC (e.g. using physical PMC0 to emulate virtual PMC0).
This path is used in most common use cases.

2) Slow path

This is when the host assigned physical PMC has a different index
from the virtual PMC (e.g. using physical PMC1 to emulate virtual PMC0)
In this case, KVM needs to rewrite the PEBS records to change the
applicable counter indexes to the virtual PMC indexes, which would
otherwise contain the physical counter index written by PEBS facility,
and switch the counter reset values to the offset corresponding to
the physical counter indexes in the DS data structure.

The previous version [0] enables both fast path and slow path, which
seems a bit more complex as the first step. In this patchset, we want
to start with the fast path to get the basic guest PEBS enabled while
keeping the slow path disabled. More focused discussion on the slow
path [1] is planned to be put to another patchset in the next step.

Compared to later versions in subsequent steps, the functionality
to support host-guest PEBS both enabled and the functionality to
emulate guest PEBS when the counter is cross-mapped are missing
in this patch set (neither of these are typical scenarios).

With the basic support, the guest can retrieve the correct PEBS
information from its own PEBS records on the Ice Lake servers.
And we expect it should work when migrating to another Ice Lake
and no regression about host perf is expected.

Here are the results of pebs test from guest/host for same workload:

perf report on guest:
# Samples: 2K of event 'instructions:ppp', # Event count (approx.): 1473377250
# Overhead  Command   Shared Object      Symbol
   57.74%  br_instr  br_instr           [.] lfsr_cond
   41.40%  br_instr  br_instr           [.] cmp_end
    0.21%  br_instr  [kernel.kallsyms]  [k] __lock_acquire

perf report on host:
# Samples: 2K of event 'instructions:ppp', # Event count (approx.): 1462721386
# Overhead  Command   Shared Object     Symbol
   57.90%  br_instr  br_instr          [.] lfsr_cond
   41.95%  br_instr  br_instr          [.] cmp_end
    0.05%  br_instr  [kernel.vmlinux]  [k] lock_acquire
    Conclusion: the profiling results on the guest are similar tothat on the host.

A minimum guest kernel version may be v5.4 or a backport version
support Icelake server PEBS.

Please check more details in each commit and feel free to comment.



V5 -> V6 Changelog:
- Rebased on the latest kvm/queue tree;
- Fix a git rebase issue (Liuxiangdong);
- Adjust the patch sequence 06/07 for bisection (Liuxiangdong);

Like Xu (16):
   perf/x86/intel: Add EPT-Friendly PEBS for Ice Lake Server
   perf/x86/intel: Handle guest PEBS overflow PMI for KVM guest
   perf/x86/core: Pass "struct kvm_pmu *" to determine the guest values
   KVM: x86/pmu: Set MSR_IA32_MISC_ENABLE_EMON bit when vPMU is enabled
   KVM: x86/pmu: Introduce the ctrl_mask value for fixed counter
   KVM: x86/pmu: Add IA32_PEBS_ENABLE MSR emulation for extended PEBS
   KVM: x86/pmu: Reprogram PEBS event to emulate guest PEBS counter
   KVM: x86/pmu: Add IA32_DS_AREA MSR emulation to support guest DS
   KVM: x86/pmu: Add PEBS_DATA_CFG MSR emulation to support adaptive PEBS
   KVM: x86: Set PEBS_UNAVAIL in IA32_MISC_ENABLE when PEBS is enabled
   KVM: x86/pmu: Adjust precise_ip to emulate Ice Lake guest PDIR counter
   KVM: x86/pmu: Move pmc_speculative_in_use() to arch/x86/kvm/pmu.h
   KVM: x86/pmu: Disable guest PEBS temporarily in two rare situations
   KVM: x86/pmu: Add kvm_pmu_cap to optimize perf_get_x86_pmu_capability
   KVM: x86/cpuid: Refactor host/guest CPU model consistency check
   KVM: x86/pmu: Expose CPUIDs feature bits PDCM, DS, DTES64

  arch/x86/events/core.c            |   5 +-
  arch/x86/events/intel/core.c      | 129 ++++++++++++++++++++++++------
  arch/x86/events/perf_event.h      |   5 +-
  arch/x86/include/asm/kvm_host.h   |  16 ++++
  arch/x86/include/asm/msr-index.h  |   6 ++
  arch/x86/include/asm/perf_event.h |   5 +-
  arch/x86/kvm/cpuid.c              |  24 ++----
  arch/x86/kvm/cpuid.h              |   5 ++
  arch/x86/kvm/pmu.c                |  50 +++++++++---
  arch/x86/kvm/pmu.h                |  38 +++++++++
  arch/x86/kvm/vmx/capabilities.h   |  26 ++++--
  arch/x86/kvm/vmx/pmu_intel.c      | 115 +++++++++++++++++++++-----
  arch/x86/kvm/vmx/vmx.c            |  24 +++++-
  arch/x86/kvm/vmx/vmx.h            |   2 +-
  arch/x86/kvm/x86.c                |  14 ++--
  15 files changed, 368 insertions(+), 96 deletions(-)