[Bug 1884095] [NEW] QEMU not sufficiently focused on qEMUlation, with resulting holes in TCG emulation coverage

[Ronald Antony]

Public bug reported:

It seems that QEMU has stopped emphasizing the EMU part of the name, and
is too much focused on virtualization.

My interest is at running legacy operating systems, and as such, they must run 
on foreign CPU platforms. m68 on intel, intel on ARM, etc.
Time doesn't stand still, and reliance on KVM and similar x86-on-x86 tricks, 
which allow the delegation of certain CPU features to the host CPU is going to 
not work going forward.

If the rumored transition of Apple to ARM is going to take place, people
will want to e.g. emulate for testing or legacy purposes a variety of
operating systems, incl. NeXTSTEP, Windows, earlier versions of MacOS on
ARM Macs.

Testing that scenario, i.e. macOS on an ARM board with the lowest
possible CPU capable of running modern macOS, results in these problems
(and of course utter failure achieving the goal):

qemu-system-x86_64: warning: TCG doesn't support requested feature: 
CPUID.01H:ECX.fma [bit 12]
qemu-system-x86_64: warning: TCG doesn't support requested feature: 
CPUID.01H:ECX.avx [bit 28]
qemu-system-x86_64: warning: TCG doesn't support requested feature: 
CPUID.07H:EBX.avx2 [bit 5]
qemu-system-x86_64: warning: TCG doesn't support requested feature: 
CPUID.80000007H:EDX.invtsc [bit 8]
qemu-system-x86_64: warning: TCG doesn't support requested feature: 
CPUID.0DH:EAX.xsavec [bit 1]

And this is emulating a lowly Penryn CPU with the required CPU flags for macOS:
-cpu 
Penryn,vendor=GenuineIntel,+sse3,+sse4.2,+aes,+xsave,+avx,+xsaveopt,+xsavec,+xgetbv1,+avx2,+bmi2,+smep,+bmi1,+fma,+movbe,+invtsc

Attempting to emulate a more feature laden intel CPU results in even
more issues.

I would propose that no CPU should be considered supported unless it can
be fully handled by TCG on a non-native host. KVM, native-on-native etc.
are nice to have, but peripheral to qEMUlation when it boils down to it.
At the very least, there should be a CLEAR distinction which CPUs
require KVM to be used, and which can be fully emulated. It should not
require wasting an afternoon to figure out that an emulation attempt is
futile because TCG lacks essential functionality.

** Affects: qemu
     Importance: Undecided
         Status: New


** Tags: emulation incomplete tcg

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https://bugs.launchpad.net/bugs/1884095

Title:
  QEMU not sufficiently focused on qEMUlation, with resulting holes in
  TCG emulation coverage

Status in QEMU:
  New

Bug description:
  It seems that QEMU has stopped emphasizing the EMU part of the name,
  and is too much focused on virtualization.

  My interest is at running legacy operating systems, and as such, they must 
run on foreign CPU platforms. m68 on intel, intel on ARM, etc.
  Time doesn't stand still, and reliance on KVM and similar x86-on-x86 tricks, 
which allow the delegation of certain CPU features to the host CPU is going to 
not work going forward.

  If the rumored transition of Apple to ARM is going to take place,
  people will want to e.g. emulate for testing or legacy purposes a
  variety of operating systems, incl. NeXTSTEP, Windows, earlier
  versions of MacOS on ARM Macs.

  Testing that scenario, i.e. macOS on an ARM board with the lowest
  possible CPU capable of running modern macOS, results in these
  problems (and of course utter failure achieving the goal):

  qemu-system-x86_64: warning: TCG doesn't support requested feature: 
CPUID.01H:ECX.fma [bit 12]
  qemu-system-x86_64: warning: TCG doesn't support requested feature: 
CPUID.01H:ECX.avx [bit 28]
  qemu-system-x86_64: warning: TCG doesn't support requested feature: 
CPUID.07H:EBX.avx2 [bit 5]
  qemu-system-x86_64: warning: TCG doesn't support requested feature: 
CPUID.80000007H:EDX.invtsc [bit 8]
  qemu-system-x86_64: warning: TCG doesn't support requested feature: 
CPUID.0DH:EAX.xsavec [bit 1]

  And this is emulating a lowly Penryn CPU with the required CPU flags for 
macOS:
  -cpu 
Penryn,vendor=GenuineIntel,+sse3,+sse4.2,+aes,+xsave,+avx,+xsaveopt,+xsavec,+xgetbv1,+avx2,+bmi2,+smep,+bmi1,+fma,+movbe,+invtsc

  Attempting to emulate a more feature laden intel CPU results in even
  more issues.

  I would propose that no CPU should be considered supported unless it
  can be fully handled by TCG on a non-native host. KVM, native-on-
  native etc. are nice to have, but peripheral to qEMUlation when it
  boils down to it. At the very least, there should be a CLEAR
  distinction which CPUs require KVM to be used, and which can be fully
  emulated. It should not require wasting an afternoon to figure out
  that an emulation attempt is futile because TCG lacks essential
  functionality.

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