[RFC PATCH v2 0/3] KVM: x86: add per-vCPU exits disable capability

[Kechen Lu <kechenl@nvidia.com>]

Summary
===========
Introduce support of vCPU-scoped ioctl with KVM_CAP_X86_DISABLE_EXITS
cap for disabling exits to enable finer-grained VM exits disabling
on per vCPU scales instead of whole guest. This patch series enabled
the vCPU-scoped exits control on HLT VM-exits.

Motivation
============
In use cases like Windows guest running heavy CPU-bound
workloads, disabling HLT VM-exits could mitigate host sched ctx switch
overhead. Simply HLT disabling on all vCPUs could bring
performance benefits, but if no pCPUs reserved for host threads, could
happened to the forced preemption as host does not know the time to do
the schedule for other host threads want to run. With this patch, we
could only disable part of vCPUs HLT exits for one guest, this still
keeps performance benefits, and also shows resiliency to host stressing
workload running at the same time.

Performance and Testing
=========================
In the host stressing workload experiment with Windows guest heavy
CPU-bound workloads, it shows good resiliency and having the ~3%
performance improvement. E.g. Passmark running in a Windows guest
with this patch disabling HLT exits on only half of vCPUs still
showing 2.4% higher main score v/s baseline.

Tested everything on AMD machines.


v1->v2 (Sean Christopherson) :
- Add explicit restriction for VM-scoped exits disabling to be called
  before vCPUs creation (patch 1)
- Use vCPU ioctl instead of 64bit vCPU bitmask (patch 3), and make exits
  disable flags check purely for vCPU instead of VM (patch 2)


Best Regards,
Kechen

Kechen Lu (3):
  KVM: x86: only allow exits disable before vCPUs created
  KVM: x86: move ()_in_guest checking to vCPU scope
  KVM: x86: add vCPU ioctl for HLT exits disable capability

 Documentation/virt/kvm/api.rst     |  4 +++-
 arch/x86/include/asm/kvm-x86-ops.h |  1 +
 arch/x86/include/asm/kvm_host.h    |  7 +++++++
 arch/x86/kvm/cpuid.c               |  2 +-
 arch/x86/kvm/lapic.c               |  2 +-
 arch/x86/kvm/svm/svm.c             | 20 +++++++++++++++-----
 arch/x86/kvm/vmx/vmx.c             | 26 ++++++++++++++++++--------
 arch/x86/kvm/x86.c                 | 24 +++++++++++++++++++++++-
 arch/x86/kvm/x86.h                 | 16 ++++++++--------
 9 files changed, 77 insertions(+), 25 deletions(-)

-- 
2.30.2