[Bug 1818075] Re: qemu x86 TCG doesn't support AVX insns

[Daniel Berrange <1818075@bugs.launchpad.net>]

QEMU, like most open source projects, relies on contributors who have
motivation, skills and available time to work on implementing particular
features. They naturally tend to focus on features that result in the
greatest benefit to their own use cases. Thus simply declaring that an
open source project, must support something won't magically make it
happen.

IOW, the lack of coverage of newer x86 instructions is largely a
reflection of the relative priorities of the current pool of
contributors and where/what they feel are the best places/features to
spend their time on.

If any person does want to work on improving x86 TCG though, the project
would happily receive patches, and existing contributors can offer
guidance & advice along the way to help get to a successful outcome.

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https://bugs.launchpad.net/bugs/1818075

Title:
  qemu x86 TCG doesn't support AVX insns

Status in QEMU:
  New

Bug description:
  I'm trying to execute code that has been built with -march=skylake
  -mtune=generic -mavx2 under qemu-user x86-64 with -cpu Skylake-Client.
  However this code just hangs at 100% CPU.

  Adding input tracing shows that it is likely hanging when dealing with
  an AVX instruction:

  warning: TCG doesn't support requested feature: CPUID.01H:ECX.fma [bit 12]
  warning: TCG doesn't support requested feature: CPUID.01H:ECX.pcid [bit 17]
  warning: TCG doesn't support requested feature: CPUID.01H:ECX.x2apic [bit 21]
  warning: TCG doesn't support requested feature: CPUID.01H:ECX.tsc-deadline [bit 24]
  warning: TCG doesn't support requested feature: CPUID.01H:ECX.avx [bit 28]
  warning: TCG doesn't support requested feature: CPUID.01H:ECX.f16c [bit 29]
  warning: TCG doesn't support requested feature: CPUID.01H:ECX.rdrand [bit 30]
  warning: TCG doesn't support requested feature: CPUID.07H:EBX.hle [bit 4]
  warning: TCG doesn't support requested feature: CPUID.07H:EBX.avx2 [bit 5]
  warning: TCG doesn't support requested feature: CPUID.07H:EBX.invpcid [bit 10]
  warning: TCG doesn't support requested feature: CPUID.07H:EBX.rtm [bit 11]
  warning: TCG doesn't support requested feature: CPUID.07H:EBX.rdseed [bit 18]
  warning: TCG doesn't support requested feature: CPUID.80000001H:ECX.3dnowprefetch [bit 8]
  warning: TCG doesn't support requested feature: CPUID.0DH:EAX.xsavec [bit 1]

  IN:
  0x4000b4ef3b:  c5 fb 5c ca              vsubsd   %xmm2, %xmm0, %xmm1
  0x4000b4ef3f:  c4 e1 fb 2c d1           vcvttsd2si %xmm1, %rdx
  0x4000b4ef44:  4c 31 e2                 xorq     %r12, %rdx
  0x4000b4ef47:  48 85 d2                 testq    %rdx, %rdx
  0x4000b4ef4a:  79 9e                    jns      0x4000b4eeea

  [ hangs ]

  Attaching a gdb produces this stacktrace:

  (gdb) bt
  #0  canonicalize (status=0x55a20ff67a88, parm=0x55a20bb807e0 <float64_params>, part=...)
      at /data/poky-tmp/master/work/x86_64-linux/qemu-native/3.1.0-r0/qemu-3.1.0/fpu/softfloat.c:350
  #1  float64_unpack_canonical (s=0x55a20ff67a88, f=0)
      at /data/poky-tmp/master/work/x86_64-linux/qemu-native/3.1.0-r0/qemu-3.1.0/fpu/softfloat.c:547
  #2  float64_sub (a=0, b=4890909195324358656, status=0x55a20ff67a88)
      at /data/poky-tmp/master/work/x86_64-linux/qemu-native/3.1.0-r0/qemu-3.1.0/fpu/softfloat.c:776
  #3  0x000055a20baa1949 in helper_subsd (env=<optimized out>, d=0x55a20ff67ad8, s=<optimized out>)
      at /data/poky-tmp/master/work/x86_64-linux/qemu-native/3.1.0-r0/qemu-3.1.0/target/i386/ops_sse.h:623
  #4  0x000055a20cfcfea8 in static_code_gen_buffer ()
  #5  0x000055a20ba3f764 in cpu_tb_exec (itb=<optimized out>, cpu=0x55a20cea2180 <static_code_gen_buffer+15684720>)
      at /data/poky-tmp/master/work/x86_64-linux/qemu-native/3.1.0-r0/qemu-3.1.0/accel/tcg/cpu-exec.c:171
  #6  cpu_loop_exec_tb (tb_exit=<synthetic pointer>, last_tb=<synthetic pointer>, tb=<optimized out>,
      cpu=0x55a20cea2180 <static_code_gen_buffer+15684720>)
      at /data/poky-tmp/master/work/x86_64-linux/qemu-native/3.1.0-r0/qemu-3.1.0/accel/tcg/cpu-exec.c:615
  #7  cpu_exec (cpu=cpu@entry=0x55a20ff5f4d0)
      at /data/poky-tmp/master/work/x86_64-linux/qemu-native/3.1.0-r0/qemu-3.1.0/accel/tcg/cpu-exec.c:725
  #8  0x000055a20ba6d728 in cpu_loop (env=0x55a20ff67780)
      at /data/poky-tmp/master/work/x86_64-linux/qemu-native/3.1.0-r0/qemu-3.1.0/linux-user/x86_64/../i386/cpu_loop.c:93
  #9  0x000055a20ba049ff in main (argc=<optimized out>, argv=0x7ffc58572868, envp=<optimized out>)
      at /data/poky-tmp/master/work/x86_64-linux/qemu-native/3.1.0-r0/qemu-3.1.0/linux-user/main.c:819

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