[PATCH v4 0/3] Allow userspace to change ID_AA64PFR1_EL1

[Shaoqin Huang]

Hi guys,

This is another try to allow userspace to change ID_AA64PFR1_EL1, and we want to
give userspace the ability to control the visible feature set for a VM, which
could be used by userspace in such a way to transparently migrate VMs.

The patch series have three part:

The first patch disable those fields which KVM doesn't know how to handle, so
KVM will only expose value 0 of those fields to the guest.

The second patch allow userspace to change ID_AA64PFR1_EL1, it allow as much as
possible fields to be writable, except some special fields which is still not
writable.

The third patch adds the kselftest to test if userspace can change the
ID_AA64PFR1_EL1.

Besides, I also noticed there is another patch [1] which try to make the
ID_AA64PFR1_EL1 writable. This patch [1] is try to enable GCS on baremental, and
add GCS support for the guest. What I understand is if we have GCS support on
baremental, it will be clear to how to handle them in KVM. And same for other
fields like NMI, THE, DF2, MTEX..

I'm still not confident about the correctness of this patch series, but I've try
my best to understand each of the fields. And follow Marc's comments to tweak
this patch series.

The question confuse me a lot is that should we allow those fields (NMI, GCS,
THE, DF2, MTEX..) which KVM doesn't know how to handle writable? Baremental
doesn't know about them, and the ftr_id_aa64pfr1[] doesn't know about them. I
follow the comment "I should handle all 15 fields", so I allow them writable
because they're disabled in the register read accessor, and their value will
alwyas be 0, the userspace can write to it but only value 0.

If I did anything wrong, please point me out. Thanks a lot.

[1] [PATCH v9 13/39] KVM: arm64: Manage GCS registers for guests
    https://lore.kernel.org/all/20240625-arm64-gcs-v9-13-0f634469b8f0@kernel.org/

Changelog:
----------
v3 -> v4:
  * Add a new patch to disable some feature which KVM doesn't know how to
    handle in the register accessor.
  * Handle all the fields in the register.
  * Fixes a small cnt issue in kselftest.

v2 -> v3:
  * Give more description about why only part of the fields can be writable.
  * Updated the writable mask by referring the latest ARM spec.

v1 -> v2:
  * Tackling the full register instead of single field.
  * Changing the patch title and commit message.

RFCv1 -> v1:
  * Fix the compilation error.
  * Delete the machine specific information and make the description more
    generable.

RFCv1: https://lore.kernel.org/all/20240612023553.127813-1-shahuang@redhat.com/
v1: https://lore.kernel.org/all/20240617075131.1006173-1-shahuang@redhat.com/
v2: https://lore.kernel.org/all/20240618063808.1040085-1-shahuang@redhat.com/
v3: https://lore.kernel.org/all/20240628060454.1936886-2-shahuang@redhat.com/

Shaoqin Huang (3):
  KVM: arm64: Disable fields that KVM doesn't know how to handle in
    ID_AA64PFR1_EL1
  KVM: arm64: Allow userspace to change ID_AA64PFR1_EL1
  KVM: selftests: aarch64: Add writable test for ID_AA64PFR1_EL1

 arch/arm64/kvm/sys_regs.c                     | 13 ++++++++++-
 .../selftests/kvm/aarch64/set_id_regs.c       | 23 ++++++++++++++++---
 2 files changed, 32 insertions(+), 4 deletions(-)

-- 
2.40.1