Malicious guests can cause bus locks to degrade the performance of a
system. Non-WB (write-back) and misaligned locked RMW
(read-modify-write) instructions are referred to as "bus locks" and
require system wide synchronization among all processors to guarantee
the atomicity. The bus locks can impose notable performance penalties
for all processors within the system.
Support for the Bus Lock Threshold is indicated by CPUID
Fn8000_000A_EDX[29] BusLockThreshold=1, the VMCB provides a Bus Lock
Threshold enable bit and an unsigned 16-bit Bus Lock Threshold count.
VMCB intercept bit
VMCB Offset Bits Function
14h 5 Intercept bus lock operations
Bus lock threshold count
VMCB Offset Bits Function
120h 15:0 Bus lock counter
During VMRUN, the bus lock threshold count is fetched and stored in an
internal count register. Prior to executing a bus lock within the
guest, the processor verifies the count in the bus lock register. If
the count is greater than zero, the processor executes the bus lock,
reducing the count. However, if the count is zero, the bus lock
operation is not performed, and instead, a Bus Lock Threshold #VMEXIT
is triggered to transfer control to the Virtual Machine Monitor (VMM).
A Bus Lock Threshold #VMEXIT is reported to the VMM with VMEXIT code
0xA5h, VMEXIT_BUSLOCK. EXITINFO1 and EXITINFO2 are set to 0 on
a VMEXIT_BUSLOCK. On a #VMEXIT, the processor writes the current
value of the Bus Lock Threshold Counter to the VMCB.
More details about the Bus Lock Threshold feature can be found in AMD
APM [1].
Patches are prepared on kvm-x86/svm (704ec48fc2fb)
Testing done:
- Added a selftest for the Bus Lock Threadshold functionality.
- Tested the Bus Lock Threshold functionality on SEV and SEV-ES guests.
- Tested the Bus Lock Threshold functionality on nested guests.
Qemu changes can be found on:
Repo: https://github.com/AMDESE/qemu.git
Branch: buslock_threshold
Qemu commandline to use the bus lock threshold functionality:
qemu-system-x86_64 -enable-kvm -cpu EPYC-Turin,+svm -M q35,bus-lock-ratelimit=10 \ ..
[1]: AMD64 Architecture Programmer's Manual Pub. 24593, April 2024,
Vol 2, 15.14.5 Bus Lock Threshold.
https://bugzilla.kernel.org/attachment.cgi?id=306250
Manali Shukla (2):
x86/cpufeatures: Add CPUID feature bit for the Bus Lock Threshold
KVM: x86: nSVM: Implement support for nested Bus Lock Threshold
Nikunj A Dadhania (2):
KVM: SVM: Enable Bus lock threshold exit
KVM: selftests: Add bus lock exit test
arch/x86/include/asm/cpufeatures.h | 1 +
arch/x86/include/asm/svm.h | 5 +-
arch/x86/include/uapi/asm/svm.h | 2 +
arch/x86/kvm/governed_features.h | 1 +
arch/x86/kvm/svm/nested.c | 25 ++++
arch/x86/kvm/svm/svm.c | 48 ++++++++
arch/x86/kvm/svm/svm.h | 1 +
arch/x86/kvm/x86.h | 1 +
tools/testing/selftests/kvm/Makefile | 1 +
.../selftests/kvm/x86_64/svm_buslock_test.c | 114 ++++++++++++++++++
10 files changed, 198 insertions(+), 1 deletion(-)
create mode 100644 tools/testing/selftests/kvm/x86_64/svm_buslock_test.c
base-commit: 704ec48fc2fbd4e41ec982662ad5bf1eee33eeb2
--
2.34.1
