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 --- 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(-) -- 2.17.1
