mainlining shenanigans
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Vitaly Kuznetsov 49097762fa Revert "KVM: VMX: Micro-optimize vmexit time when not exposing PMU"
Guest crashes are observed on a Cascade Lake system when 'perf top' is
launched on the host, e.g.

 BUG: unable to handle kernel paging request at fffffe0000073038
 PGD 7ffa7067 P4D 7ffa7067 PUD 7ffa6067 PMD 7ffa5067 PTE ffffffffff120
 Oops: 0000 [#1] SMP PTI
 CPU: 1 PID: 1 Comm: systemd Not tainted 4.18.0+ #380
...
 Call Trace:
  serial8250_console_write+0xfe/0x1f0
  call_console_drivers.constprop.0+0x9d/0x120
  console_unlock+0x1ea/0x460

Call traces are different but the crash is imminent. The problem was
blindly bisected to the commit 041bc42ce2 ("KVM: VMX: Micro-optimize
vmexit time when not exposing PMU"). It was also confirmed that the
issue goes away if PMU is exposed to the guest.

With some instrumentation of the guest we can see what is being switched
(when we do atomic_switch_perf_msrs()):

 vmx_vcpu_run: switching 2 msrs
 vmx_vcpu_run: switching MSR38f guest: 70000000d host: 70000000f
 vmx_vcpu_run: switching MSR3f1 guest: 0 host: 2

The current guess is that PEBS (MSR_IA32_PEBS_ENABLE, 0x3f1) is to blame.
Regardless of whether PMU is exposed to the guest or not, PEBS needs to
be disabled upon switch.

This reverts commit 041bc42ce2.

Reported-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20200619094046.654019-1-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
2020-06-19 08:13:40 -04:00
arch Revert "KVM: VMX: Micro-optimize vmexit time when not exposing PMU" 2020-06-19 08:13:40 -04:00
block Kbuild updates for v5.8 (2nd) 2020-06-13 13:29:16 -07:00
certs .gitignore: add SPDX License Identifier 2020-03-25 11:50:48 +01:00
crypto Merge branch 'rwonce/rework' of git://git.kernel.org/pub/scm/linux/kernel/git/will/linux 2020-06-10 14:46:54 -07:00
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include Add additional LSM hooks for SafeSetID 2020-06-14 11:39:31 -07:00
init Kbuild updates for v5.8 (2nd) 2020-06-13 13:29:16 -07:00
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usr bpfilter: match bit size of bpfilter_umh to that of the kernel 2020-05-17 18:52:01 +09:00
virt MIPS: 2020-06-12 11:05:52 -07:00
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.cocciconfig
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.gitignore modpost: generate vmlinux.symvers and reuse it for the second modpost 2020-06-06 23:38:12 +09:00
.mailmap A fair amount of stuff this time around, dominated by yet another massive 2020-06-01 15:45:27 -07:00
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Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS Merge branch 'i2c/for-5.8' of git://git.kernel.org/pub/scm/linux/kernel/git/wsa/linux 2020-06-13 13:12:38 -07:00
Makefile Linux 5.8-rc1 2020-06-14 12:45:04 -07:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.