[PATCH v2 0/5] Handle monitor trap flag during instruction emulation

Oliver Upton
2020-01-28T09:27:30Z

v1: http://lore.kernel.org/r/20200113221053.22053-1-oupton@google.com

v1 => v2:
- Don't split the #DB delivery by vendors. Unconditionally injecting
#DB payloads into the 'pending debug exceptions' field will cause KVM
to get stuck in a loop. Per the SDM, when hardware injects an event
resulting from this field's value, it is checked against the
exception interception bitmap.
- Address Sean's comments by injecting the VM-exit into L1 from
vmx_check_nested_events().
- Added fix for nested INIT VM-exits + 'pending debug exceptions' field
as it was noticed in implementing v2.
- Drop Peter + Jim's Reviewed-by tags, as the patch set has changed
since v1.

KVM already provides guests the ability to use the 'monitor trap flag'
VM-execution control. Support for this flag is provided by the fact that
KVM unconditionally forwards MTF VM-exits to the guest (if requested),
as KVM doesn't utilize MTF. While this provides support during hardware
instruction execution, it is insufficient for instruction emulation.

Should L0 emulate an instruction on the behalf of L2, L0 should also
synthesize an MTF VM-exit into L1, should control be set.

The first patch corrects a nuanced difference between the definition of
a #DB exception payload field and DR6 register. Mask off bit 12 which is
defined in the 'pending debug exceptions' field when applying to DR6,
since the payload field is said to be compatible with the aforementioned
VMCS field.

The second patch sets the 'pending debug exceptions' VMCS field when
delivering an INIT signal VM-exit to L1, as described in the SDM. This
patch also introduces helpers for setting the 'pending debug exceptions'
VMCS field.

The third patch massages KVM's handling of exception payloads with
regard to API compatibility. Rather than immediately injecting the
payload w/o opt-in, instead defer the payload + immediately inject
before completing a KVM_GET_VCPU_EVENTS. This maintains API
compatibility whilst correcting #DB behavior with regard to higher
priority VM-exit events.

Fourth patch introduces MTF implementation for emulated instructions.
Identify if an MTF is due on an instruction boundary from
kvm_vcpu_do_singlestep(), however only deliver this VM-exit from
vmx_check_nested_events() to respect the relative prioritization to
other VM-exits. Since this augments the nested state, introduce a new
flag for (de)serialization.

Last patch adds tests to kvm-unit-tests to assert the correctness of MTF
under several conditions (concurrent #DB trap, #DB fault, etc). These
tests pass under virtualization with this series as well as on
bare-metal.

Oliver Upton (4):
KVM: x86: Mask off reserved bit from #DB exception payload
KVM: nVMX: Handle pending #DB when injecting INIT VM-exit
KVM: x86: Deliver exception payload on KVM_GET_VCPU_EVENTS
KVM: nVMX: Emulate MTF when performing instruction emulation

arch/x86/include/asm/kvm_host.h | 1 +
arch/x86/include/uapi/asm/kvm.h | 1 +
arch/x86/kvm/svm.c | 1 +
arch/x86/kvm/vmx/nested.c | 60 ++++++++++++++++++++++++++++++++-
arch/x86/kvm/vmx/nested.h | 5 +++
arch/x86/kvm/vmx/vmx.c | 22 ++++++++++++
arch/x86/kvm/vmx/vmx.h | 3 ++
arch/x86/kvm/x86.c | 52 +++++++++++++++++-----------
8 files changed, 125 insertions(+), 20 deletions(-)

--
2.25.0.341.g760bfbb309-goog