[Qemu-devel] [Bug 1824768] Re: Qemu ARMv7 TCG MultiThreading for i386 guest doesn't work

[Peter Maydell]

Implementing a stronger guest memory model than the host has is tricky
-- we would need to emit barrier instructions after each guest load or
store, which would slow down straight-line execution speed by quite a
lot, perhaps by so much that it would outweigh the gain from being able
to run more than one thread at once. (Perhaps some of the armv8 load-
acquire/store-release insns would help, but they're no good to you on a
v7 CPU.)

There aren't many people overall who want to try to run emulation on
anything other than x86 host.

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https://bugs.launchpad.net/bugs/1824768

Title:
  Qemu ARMv7 TCG MultiThreading for i386 guest doesn't work

Status in QEMU:
  New

Bug description:
  Using any Linux image (in this case Alpine Linux iso) I want to
  utilise all cores of my Raspberry with --accel,thread=multi. I know
  there is a probably still problem with memory ordering of the host,
  but I have also seen some very old commits which could potentially
  help with it.

  But anyway, with version qemu-i386 version 3.1.0 (Debian 1:3.1+dfsg-7)
  I can see OpenRC starting up services and then the kernel crash.

  With version QEMU emulator version 3.1.93 (v4.0.0-rc3-dirty)
  The whole machine crash with this error:
  Illegal instruction

  Using command:
  ./qemu-system-i386 -cdrom alpine.iso --accel tcg,thread=multi

  Full Console Output:
  qemu-system-i386: warning: Guest expects a stronger memory ordering than the 
host provides
  This may cause strange/hard to debug errors
  Illegal instruction

  Kernel:
  Linux raspberrypi 4.14.98-v7+ #1200 SMP Tue Feb 12 20:27:48 GMT 2019 armv7l 
GNU/Linux

  CPU:
  ARMv7 Processor rev 5 (v7l)
  Features: half thumb fastmult vfp edsp neon vfpv3 tls vfpv4 idiva idivt 
vfpd32 lpae evtstrm
  4 cores

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