Re: [PATCH v5 4/5] KVM: SVM: Add support for KVM_CAP_X86_BUS_LOCK_EXIT on SVM CPUs

On 02-05-2025 10:33, Manali Shukla wrote:

• /*

• * Stash vmcb02's counter if the guest hasn't moved past the guilty
  

• * instrution; otherwise, reset the counter to '0'.
  

typo instrution -> instruction

• *
  

• * In order to detect if L2 has made forward progress or not, track the
  

• * RIP at which a bus lock has occurred on a per-vmcb12 basis.  If RIP
  

• * is changed, guest has clearly made forward progress, bus_lock_counter
  

• * still remained '1', so reset bus_lock_counter to '0'. Eg. In the
  

• * scenario, where a buslock happened in L1 before VMRUN, the bus lock
  

• * firmly happened on an instruction in the past. Even if vmcb01's
  

• * counter is still '1', (because the guilty instruction got patched),
  

• * the vCPU has clearly made forward progress and so KVM should reset
  

• * vmcb02's counter to '0'.
  

• *
  

• * If the RIP hasn't changed, stash the bus lock counter at nested VMRUN
  

• * to prevent the same guilty instruction from triggering a VM-Exit. Eg.
  

• * if userspace rate-limits the vCPU, then it's entirely possible that
  

• * L1's tick interrupt is pending by the time userspace re-runs the
  

• * vCPU.  If KVM unconditionally clears the counter on VMRUN, then when
  

• * L1 re-enters L2, the same instruction will trigger a VM-Exit and the
  

• * entire cycle start over.
  

• */
  

Thanks, Alok