[PATCH 0/4] KVM: x86: Let the guest write to multiple debug registers with one vmexit

[Paolo Bonzini <pbonzini@redhat.com>]

Alex Williamson reported that a Windows game does something weird that
makes the guest save and restore debug registers on each context switch.
This cause several hundred thousands vmexits per second, and basically
cuts performance in half when running under KVM.

However, when not running in guest-debug mode, the guest controls the
debug registers and having to take an exit for each DR access is a waste
of time.  We just need one vmexit to load any stale values of DR0-DR6,
and then we can let the guest run freely.  On the next vmexit (whatever
the reason) we will read out whatever changes the guest made to the
debug registers.

On top of this, we can implement SVM support and let nested guests run
with dirty debug registers too.

Paolo Bonzini (4):
  KVM: vmx: we do rely on loading DR7 on entry
  KVM: x86: change vcpu->arch.switch_db_regs to a bit mask
  KVM: x86: Allow the guest to run with dirty debug registers
  KVM: vmx: Allow the guest to run with dirty debug registers

 arch/x86/include/asm/kvm_host.h |  8 ++++++-
 arch/x86/kvm/vmx.c              | 48 +++++++++++++++++++++++++++++++++++++----
 arch/x86/kvm/x86.c              | 29 +++++++++++++++++++++++--
 3 files changed, 78 insertions(+), 7 deletions(-)

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1.8.3.1