kvm_target, QEMU_KVM_ARM_TARGET_GENERIC_V8 questions

[Leif Lindholm <leif@nuviainc.com>]

Hi there,

(all this done on current HEAD: 66234fee9c)

I was looking through the definition of the aarch64 "max" cpu, and
noticed it invokes aarch64_a57_initfn as a template, followed by
overriding some feature and ID fields to enable additional
functionality.

I then noticed it does not override kvm_target, which hence remains
set as QEMU_KVM_ARM_TARGET_CORTEX_A57. Of course, this only happens on
the else side of the if (kvm_enabled()) branch, so doesn't affect it.

However, while looking at this, I noticed aarch64_a72_initfn doesn't
initialise kvm_target at all.

Then I looked at the definition of QEMU_KVM_ARM_TARGET_CORTEX_A57,
found there was also a KVM_ARM_TARGET_CORTEX_A57, and then I noticed
there exists a KVM_ARM_TARGET_CORTEX_A57 (in
linux-headers/asm-arm64/kvm.h), and *then* I noticed there exists a
KVM_ARM_TARGET_GENERIC_V8 definition there as well - plus a comment
saying "please don't add any more targets unless you really need to".
Then I noticed there isn't a corresponding
QEMU_KVM_ARM_TARGET_GENERIC_V8 in target/arm/kvm-consts.h.

So, then I decided to actually test things, and found that
(with -enable-kvm):
- on Cortex-A53 hardware
  - "max" kvm_target gets initialized to 4 (KVM_ARM_TARGET_CORTEX_A53)
    by kvm_arm_get_host_cpu_features (as returned from the kernel for
    vm_arm_create_scratch_host_vcpu)
  - cortex-A72 fails to start with "KVM is not supported for this guest
    CPU type"
  (fair enough, it's later than A53)
- on Cortex-A72 hardware
  - "max" kvm_target gets initialized to 5 (KVM_ARM_TARGET_GENERIC_V8)
    by kvm_arm_get_host_cpu_features
  - "cortex-A72" fails to start (umm...)

However ... if I haven't managed to confuse myself somewhere in here
(which is completely possible), would it be OK if I submitted a set of
patches that:
- add a QEMU_KVM_ARM_TARGET_GENERIC_V8 to match the kernel one
- set kvm_target for Cortex-A72 to QEMU_KVM_ARM_TARGET_GENERIC_V8
  - alternatively drop the explicit settings for A57/A53
- drop the call from aarch64_max_initfn to aarch64_a57_initfn, and
  copy the relevant bits into the former for the !kvm case

?

/
    Leif