[Qemu-devel] [qom-cpu PATCH 0/3] target-i386: Make most CPU models work with "enforce" out of the box

[Eduardo Habkost <ehabkost@redhat.com>]

Most of the bits that make "enforce" breaks were introduced in 2010 by commit
8560efed6a72a816c0115f41ddb9d79f7ce63f28. The intention behind that commit made
sense, the only problem is that we can't guarantee guest ABI stability across
hosts if we simply rely on trimming of CPU features based on host capabilities.

So, this series remove CPUID bits from the CPU model definitions so they become
defaults that: 1) won't unexpectly stop working when we start using the
"enforce" flag; 2) won't silently break the guest ABI when TCG or KVM start
supporting new features.

There's only one non-trivial case left: the qemu32/qemu64 models. The problem
with them is that we have conflicting expectations about it, from different
users:

TCG users expect the default CPU model to contain most TCG-supported features
(and it makes sense). See, for example, commit
f1e00a9cf326acc1f2386a72525af8859852e1df.

KVM users expect the default CPU model to be a conservative choice which will
work on most host CPUs (and will only contain features that are supported by
KVM).

We could solve the qemu32/qemu64 issue by having different defaults for TCG and
KVM. But we have existinting management code (libvirt) that already expects
qemu32 or qemu64 to be the default, and changing the default would break that
code. I will send an RFC to address that later.

Cc: Aurelien Jarno <aurelien@aurel32.net>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: kvm@vger.kernel.org

Eduardo Habkost (3):
  target-i386: Disable CPUID_ACPI by default on KVM mode
  target-i386: Remove unsupported bits from all CPU models
  target-i386: Don't enable nested VMX by default

 hw/i386/pc_piix.c |  2 ++
 hw/i386/pc_q35.c  |  2 ++
 target-i386/cpu.c | 34 +++++++++++++++++++++-------------
 3 files changed, 25 insertions(+), 13 deletions(-)

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1.9.3