Re: [RFC PATCH v1 0/4] Add support for the Bus Lock Threshold

[Manali Shukla <manali.shukla@amd.com>]

On 7/9/2024 11:21 PM, Manali Shukla wrote:
> 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

A gentle reminder.

-Manali