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(-)