Hi Xiaoyao, Thanks for reviewing my patches.
On 4/16/2025 11:44 AM, Xiaoyao Li wrote: > On 3/24/2025 9:02 PM, Manali Shukla wrote: >> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst >> index 5fe84f2427b5..f7c925aa0c4f 100644 >> --- a/Documentation/virt/kvm/api.rst >> +++ b/Documentation/virt/kvm/api.rst >> @@ -7909,6 +7909,25 @@ apply some other policy-based mitigation. When >> exiting to userspace, KVM sets >> KVM_RUN_X86_BUS_LOCK in vcpu-run->flags, and conditionally sets the >> exit_reason >> to KVM_EXIT_X86_BUS_LOCK. >> +Note! KVM_CAP_X86_BUS_LOCK_EXIT on AMD CPUs with the Bus Lock Threshold >> is close >> +enough to INTEL's Bus Lock Detection VM-Exit to allow using >> +KVM_CAP_X86_BUS_LOCK_EXIT for AMD CPUs. >> + >> +The biggest difference between the two features is that Threshold (AMD >> CPUs) is >> +fault-like i.e. the bus lock exit to user space occurs with RIP pointing at >> the >> +offending instruction, whereas Detection (Intel CPUs) is trap-like i.e. the >> bus >> +lock exit to user space occurs with RIP pointing at the instruction right >> after >> +the guilty one. >> > > >> +The bus lock threshold on AMD CPUs provides a per-VMCB counter which is >> +decremented every time a bus lock occurs, and a VM-Exit is triggered if and >> only >> +if the bus lock counter is '0'. >> + >> +To provide Detection-like semantics for AMD CPUs, the bus lock counter has >> been >> +initialized to '0', i.e. exit on every bus lock, and when re-executing the >> +guilty instruction, the bus lock counter has been set to '1' to effectively >> step >> +past the instruction. > > From the perspective of API, I don't think the last two paragraphs matter > much to userspace. > > It should describe what userspace can/should do. E.g., when exit to userspace > due to bus lock on AMD platform, the RIP points at the instruction which > causes the bus lock. Userspace can advance the RIP itself before re-enter the > guest to make progress. If userspace doesn't change the RIP, KVM internal can > handle it by making the re-execution of the instruction doesn't trigger bus > lock VM exit to allow progress. Sure. -Manali
