[RFC PATCH 0/2] KVM: VMX: Fix VM entry failure on PT_MODE_HOST_GUEST while host is using PT

Xiaoyao Li
2022-08-25T08:56:43Z

There is one bug in KVM that can hit vm-entry failure 100% on platform
supporting PT_MODE_HOST_GUEST mode following below steps:

1. #modprobe -r kvm_intel
2. #modprobe kvm_intel pt_mode=1
3. start a VM with QEMU
4. on host: #perf record -e intel_pt//

The vm-entry failure happens because it violates the requirement stated
in Intel SDM 26.2.1.1 VM-Execution Control Fields

If the logical processor is operating with Intel PT enabled (if
IA32_RTIT_CTL.TraceEn = 1) at the time of VM entry, the "load
IA32_RTIT_CTL" VM-entry control must be 0.

On PT_MODE_HOST_GUEST node, PT_MODE_HOST_GUEST is always set. Thus KVM
needs to ensure IA32_RTIT_CTL.TraceEn is 0 before VM-entry. Currently KVM
manually WRMSR(IA32_RTIT_CTL) to clear TraceEn bit. However, it doesn't
work everytime since there is a posibility that IA32_RTIT_CTL.TraceEn is
re-enabled in PT PMI handler before vm-entry. This series tries to fix
the issue by exposing two interfaces from Intel PT driver for the purose
to stop and resume Intel PT on host. It prevents PT PMI handler from
re-enabling PT. By the way, it also fixes another issue that PT PMI
touches PT MSRs whihc leads to what KVM stores for host bemomes stale.

Xiaoyao Li (2):
perf/x86/intel/pt: Introduce intel_pt_{stop,resume}()
KVM: VMX: Stop/resume host PT before/after VM entry when
PT_MODE_HOST_GUEST

arch/x86/events/intel/pt.c | 11 ++++++++++-
arch/x86/include/asm/intel_pt.h | 6 ++++--
arch/x86/kernel/crash.c | 4 ++--
arch/x86/kvm/vmx/vmx.c | 11 ++++++++++-
4 files changed, 26 insertions(+), 6 deletions(-)

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2.27.0