[PATCH v5 0/4] Allow userspace to change ID_AA64PFR1_EL1

[Shaoqin Huang]

Hi Marc, Oliver,

Kindly ping for this series. Welcome your feedbacks. I wonder if this 
implementation satisfy your expectation?

Thanks,
Shaoqin

On 7/23/24 15:19, Shaoqin Huang wrote:
> 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 four 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 check the FEAT_SSBS in guest IDREG instead of the cpu
> capability.
> 
> The third patch allow userspace to change ID_AA64PFR1_EL1, it only advertise the
> fields known to KVM and leave others unadvertise.
> 
> The fourth 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.. At that time, they can be writable.
> 
> [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:
> ----------
> v4 -> v5:
>    * Only advertise fields which KVM know how to handle to userspace, leave
>      others unadvertised.
>    * Add a new patch to check FEAT_SSBS in IDREG instead of cpu capability.
>    * Tweak the kselftest writable fields.
>    * Improve the commit message.
> 
> 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/
> v4: https://lore.kernel.org/all/20240718035017.434996-1-shahuang@redhat.com/
> 
> Shaoqin Huang (4):
>    KVM: arm64: Disable fields that KVM doesn't know how to handle in
>      ID_AA64PFR1_EL1
>    KVM: arm64: Use kvm_has_feat() to check if FEAT_SSBS is advertised to
>      the guest
>    KVM: arm64: Allow userspace to change ID_AA64PFR1_EL1
>    KVM: selftests: aarch64: Add writable test for ID_AA64PFR1_EL1
> 
>   arch/arm64/kvm/hypercalls.c                   | 12 +++++-----
>   arch/arm64/kvm/sys_regs.c                     | 22 ++++++++++++++++++-
>   .../selftests/kvm/aarch64/set_id_regs.c       | 14 +++++++++---
>   3 files changed, 38 insertions(+), 10 deletions(-)
> 

-- 
Shaoqin