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[Qemu-devel] [Bug 1774677] Re: -icount increases boot time by >10x

From: Peter Maydell
Subject: [Qemu-devel] [Bug 1774677] Re: -icount increases boot time by >10x
Date: Fri, 06 Jul 2018 13:00:55 -0000

Yes, I can reproduce this. Also, interestingly, with the -icount version
when I typed 'exit' to quit the guest I got this abort:

qemu: fatal: cpu_io_recompile: could not find TB for pc=0x7fcd0aae636e
EAX=c88053a4 EBX=00000069 ECX=c00211a4 EDX=00000000
ESI=c7812180 EDI=c78126f0 EBP=0000000f ESP=c7a0de44
EIP=c11ada30 EFL=00000287 [--S--PC] CPL=0 II=0 A20=1 SMM=0 HLT=0
ES =007b 00000000 ffffffff 00cff300 DPL=3 DS   [-WA]
CS =0060 00000000 ffffffff 00cf9a00 DPL=0 CS32 [-R-]
SS =0068 00000000 ffffffff 00c09300 DPL=0 DS   [-WA]
DS =007b 00000000 ffffffff 00cff300 DPL=3 DS   [-WA]
FS =0000 00000000 00000000 00000000
GS =0033 080a130c ffffffff 00dff300 DPL=3 DS   [-WA]
LDT=0000 00000000 00000000 00008200 DPL=0 LDT
TR =0080 c12fbe60 0000206b 00008900 DPL=0 TSS32-avl
GDT=     c12f6000 000000ff
IDT=     ffffc000 000007ff
CR0=8005003b CR2=09d40ffc CR3=07a08000 CR4=00000690
DR0=00000000 DR1=00000000 DR2=00000000 DR3=00000000 
DR6=ffff0ff0 DR7=00000400
CCS=c8800000 CCD=c8805200 CCO=SUBL    
FCW=037f FSW=0000 [ST=0] FTW=00 MXCSR=00001f80
FPR0=0000000000000000 0000 FPR1=0000000000000000 0000
FPR2=0000000000000000 0000 FPR3=0000000000000000 0000
FPR4=0000000000000000 0000 FPR5=0000000000000000 0000
FPR6=0000000000000000 0000 FPR7=ec00000000000000 4006
XMM00=00000000000000000000000000000000 XMM01=00000000000000000000000000000000
XMM02=00000000000000000000000000000000 XMM03=00000000000000000000000000000000
XMM04=00000000000000000000000000000000 XMM05=00000000000000000000000000000000
XMM06=00000000000000000000000000000000 XMM07=00000000000000000000000000000000
Aborted (core dumped)

** Changed in: qemu
       Status: New => Confirmed

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  -icount increases boot time by >10x

Status in QEMU:

Bug description:
  When I specify the -icount option, some guest operations such as
  booting a Linux kernel take more than 10 times longer than otherwise.
  For example, the following will boot Aboriginal Linux to the login
  prompt about 6 seconds on my system (using TCG, not KVM):

  gunzip <system-image-i686.tar.gz | tar xfv -
  cd system-image-i686
  sh run-emulator.sh

  If I replace the last line with

  QEMU_EXTRA="-icount shift=auto" sh run-emulator.sh

  booting to the login prompt takes about 1 minute 20 seconds.

  I have tried different values for "shift" other than the "auto" used
  above, but have not been able to find one that gives reasonable
  performance.  Specifying "sleep=off" also did not help.

  During the slow boots, qemu appears to spend most of its time
  sleeping, not using the host CPU.

  I see this with multiple versions of qemu, including current git
  sources (c181ddaa176856b3cd2dfd12bbcf25fa9c884a97), and on multiple
  host OSes, including Debian 9 on x86_64.

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