[PATCH v2 0/3] Add KVM support for bus lock debug exception

[Chenyi Qiang]

A bus lock is acquired either through split locked access to writeback
(WB) memory or by using locks to uncacheable (UC) memory. This is
typically > 1000 cycles slower than atomic opertaion within a cache
line. It also disrupts performance on other cores.

Bus lock debug exception is a sub-feature of bus lock detection. It is
an ability to notify the kernel by an #DB trap after the instruction
acquires a bus lock when CPL>0. This allows the kernel to enforce user
application throttling or mitigatioins.

Expose the bus lock debug exception to guest by the enumeration of
CPUID.(EAX=7,ECX=0).ECX[24]. Software in guest can enable these
exceptions by setting the DEBUGCTLMSR_BUS_LOCK_DETECT(bit 2) of
MSR_IA32_DEBUTCTL.

The bus lock #DB exception can also be intercepted by the VMM and
identified through the bit 11 of the exit qualification at VM exit. The
bit 11 (DR6_BUS_LOCK) of DR6 register is introduced to indicate a bus
lock #DB exception. DR6_BUS_LOCK has formerly always been 1 and delivery
of a bus lock #DB clears it. The VMM should emulate the exceptions by
clearing the bit 11 of the guest DR6.

Bus lock debug exception kernel patches are available at:
https://lore.kernel.org/lkml/20201124205245.4164633-1-fenghua.yu@xxxxxxxxx

Document for Bus Lock Detection is now available at the latest "Intel
Architecture Instruction Set Extensions Programming Reference".

Document Link:
https://software.intel.com/content/www/us/en/develop/download/intel-architecture-instruction-set-extensions-programming-reference.html

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Changelogs

v1->v2:
- rename the DR6_INIT to DR6_LOW_ACTIVE suggested by Paolo and split it
  out as a new commit.
- KVM v1:https://lore.kernel.org/lkml/20210108064924.1677-1-chenyi.qiang@xxxxxxxxx


Chenyi Qiang (3):
  KVM: X86: Rename DR6_INIT to DR6_ACTIVE_LOW
  KVM: X86: Add support for the emulation of DR6_BUS_LOCK bit
  KVM: X86: Expose bus lock debug exception to guest

 arch/x86/include/asm/kvm_host.h | 14 +++++++--
 arch/x86/kvm/cpuid.c            |  3 +-
 arch/x86/kvm/emulate.c          |  2 +-
 arch/x86/kvm/svm/nested.c       |  2 +-
 arch/x86/kvm/svm/svm.c          |  6 ++--
 arch/x86/kvm/vmx/nested.c       |  4 +--
 arch/x86/kvm/vmx/vmx.c          | 27 ++++++++++++++---
 arch/x86/kvm/x86.c              | 52 +++++++++++++++------------------
 arch/x86/kvm/x86.h              |  2 ++
 9 files changed, 69 insertions(+), 43 deletions(-)

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2.17.1