[Bug 1912777] Re: KVM_EXIT_MMIO has increased in Qemu4.0.0 when compared to Qemu 2.11.0

[ANIMESH KUMAR SINHA]

** Also affects: qemu (Ubuntu)
   Importance: Undecided
       Status: New

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Title:
  KVM_EXIT_MMIO has increased in Qemu4.0.0 when compared to Qemu 2.11.0

Status in QEMU:
  Opinion
Status in Ubuntu:
  New
Status in qemu package in Ubuntu:
  New

Bug description:
  I was able to generate trace dump in Qemu for kvm_run_exit event in both QEMU 
2.11.0 and QEMU 4.0.0
  From the trace i noticed that the number of KVM_KXIT_MMIO calls has increased 
alot and is causing delay in testcase execution.

  I executed same testcase from Qemu 2.11 and Qemu4.
  Inside Virtual machine when using qemu 2.11 testcase got completed in 11 
seconds
  but the same testcase when executed on Qemu 4.0.0 got executed in 26 seconds.

  I did a bit of digging and extracted the kvm_run_exit to figure out
  whats going on.

  Please find
  Stats from Qemu2.11:

  KVM_EXIT_UNKNOWN          : 0
  KVM_EXIT_EXCEPTION        : 0
  KVM_EXIT_IO               : 182513
  KVM_EXIT_HYPERCALL        : 0
  KVM_EXIT_DEBUG            : 0
  KVM_EXIT_HLT              : 0
  KVM_EXIT_MMIO             : 216701
  KVM_EXIT_IRQ_WINDOW_OPEN  : 0
  KVM_EXIT_SHUTDOWN         : 0
  KVM_EXIT_FAIL_ENTRY       : 0
  KVM_EXIT_INTR             : 0
  KVM_EXIT_SET_TPR          : 0
  KVM_EXIT_TPR_ACCESS       : 0
  KVM_EXIT_S390_SIEIC       : 0
  KVM_EXIT_S390_RESET       : 0
  KVM_EXIT_DCR              : 0
  KVM_EXIT_NMI              : 0
  KVM_EXIT_INTERNAL_ERROR   : 0
  KVM_EXIT_OSI              : 0
  KVM_EXIT_PAPR_HCALL       : 0
  KVM_EXIT_S390_UCONTROL    : 0
  KVM_EXIT_WATCHDOG         : 0
  KVM_EXIT_S390_TSCH        : 0
  KVM_EXIT_EPR              : 0
  KVM_EXIT_SYSTEM_EVENT     : 0
  KVM_EXIT_S390_STSI        : 0
  KVM_EXIT_IOAPIC_EOI       : 0
  KVM_EXIT_HYPERV           : 0

  KVM_RUN_EXIT              : 399214  (Total in Qemu 2.11 for a
  testcase)

  Stats For Qemu 4.0.0:

  VM_EXIT_UNKNOWN           : 0
  KVM_EXIT_EXCEPTION        : 0
  KVM_EXIT_IO               : 163729
  KVM_EXIT_HYPERCALL        : 0
  KVM_EXIT_DEBUG            : 0
  KVM_EXIT_HLT              : 0
  KVM_EXIT_MMIO             : 1094231
  KVM_EXIT_IRQ_WINDOW_OPEN  : 46
  KVM_EXIT_SHUTDOWN         : 0
  KVM_EXIT_FAIL_ENTRY       : 0
  KVM_EXIT_INTR             : 0
  KVM_EXIT_SET_TPR          : 0
  KVM_EXIT_TPR_ACCESS       : 0
  KVM_EXIT_S390_SIEIC       : 0
  KVM_EXIT_S390_RESET       : 0
  KVM_EXIT_DCR              : 0
  KVM_EXIT_NMI              : 0
  KVM_EXIT_INTERNAL_ERROR   : 0
  KVM_EXIT_OSI              : 0
  KVM_EXIT_PAPR_HCALL       : 0
  KVM_EXIT_S390_UCONTROL    : 0
  KVM_EXIT_WATCHDOG         : 0
  KVM_EXIT_S390_TSCH        : 0
  KVM_EXIT_EPR              : 0
  KVM_EXIT_SYSTEM_EVENT     : 0
  KVM_EXIT_S390_STSI        : 0
  KVM_EXIT_IOAPIC_EOI       : 464
  KVM_EXIT_HYPERV           : 0

  KVM_RUN_EXIT              : 1258470 (Total in qemu 4.0.0 for same
  testcase)

  From above analysis i found that the number of KVM_EXIT_MMIO has
  increased by 4.x.

  Could someone from qemu community help me understand as to why the
  MMIO exits have increased in qemu4 ?

  The results i obtained are after running same testcase.
  On Qemu2.11 testcase gets completed in : 11seconds
  on Qemu4.11 testcase gets completed in : 26 seconds

  VM Qcow2 used in Ubuntu 16.04
  VM kernel OS is : 4.4 generic

  Let me know incase more information is required .

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