Commit 90a8d72d authored by Vitaly Kuznetsov's avatar Vitaly Kuznetsov Committed by Sasha Levin
Browse files

Revert "KVM: VMX: Micro-optimize vmexit time when not exposing PMU"

[ Upstream commit 49097762 ]

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 041bc42c ("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 041bc42c

.
Reported-by: default avatarMaxime Coquelin <maxime.coquelin@redhat.com>
Signed-off-by: default avatarVitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20200619094046.654019-1-vkuznets@redhat.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: default avatarSasha Levin <sashal@kernel.org>
parent 9a0dc2c6
......@@ -6575,8 +6575,7 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
pt_guest_enter(vmx);
if (vcpu_to_pmu(vcpu)->version)
atomic_switch_perf_msrs(vmx);
atomic_switch_perf_msrs(vmx);
atomic_switch_umwait_control_msr(vmx);
if (enable_preemption_timer)
......
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