[PATCH v4 2/3] KVM: arm64: Allow userspace to change ID_AA64PFR1_EL1

[Shaoqin Huang]

Allow userspace to change the guest-visible value of the register with
different way of handling:

  - Since the RAS and MPAM is not writable in the ID_AA64PFR0_EL1
    register, RAS_frac and MPAM_frac are also not writable in the
    ID_AA64PFR1_EL1 register.

  - The MTE is controlled by an internal flag (KVM_ARCH_FLAG_MTE_ENABLED),
    so it's not writable.

  - For those fields which KVM doesn't know how to handle, they have
    are not exposed to the guest (being disabled in the register read
    accessor), those fields value will always be 0. Allow those fields
    writable is fine, since the userspace can only write 0 into those
    fields. Maybe in the future KVM know how to handle some of the
    fields, then they can be written into other value.
    So let them writable.
    Those fields include SME, RNDR_trap, NMI, GCS, THE, DF2, PFAR,
    MTE_frac, MTEX.

  - The BT, SSBS, CSV2_frac don't introduce any new registers which KVM
    doesn't know how to handle, they can be written without ill effect.
    So let them writable.

Besides, we don't do the crosscheck in KVM about the CSV2_frac even if
it depends on the value of CSV2, it should be made sure by the VMM
instead of KVM.

Signed-off-by: Shaoqin Huang <shahuang at redhat.com>
---
 arch/arm64/kvm/sys_regs.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 4508288b9d38..26b38165a8be 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -2314,7 +2314,10 @@ static const struct sys_reg_desc sys_reg_descs[] = {
 		   ID_AA64PFR0_EL1_GIC |
 		   ID_AA64PFR0_EL1_AdvSIMD |
 		   ID_AA64PFR0_EL1_FP), },
-	ID_SANITISED(ID_AA64PFR1_EL1),
+	ID_WRITABLE(ID_AA64PFR1_EL1, ~(ID_AA64PFR1_EL1_RES0 |
+				       ID_AA64PFR1_EL1_MPAM_frac |
+				       ID_AA64PFR1_EL1_RAS_frac |
+				       ID_AA64PFR1_EL1_MTE)),
 	ID_UNALLOCATED(4,2),
 	ID_UNALLOCATED(4,3),
 	ID_WRITABLE(ID_AA64ZFR0_EL1, ~ID_AA64ZFR0_EL1_RES0),
-- 
2.40.1