Allow userspace to downgrade VH in ID_AA64MMFR1_EL1. Userspace can
only change this value from high to low.

Signed-off-by: Jinqian Yang <yangjinqian1@huawei.com>
---
 arch/arm64/kvm/sys_regs.c                       | 1 -
 tools/testing/selftests/kvm/arm64/set_id_regs.c | 1 +
 2 files changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index db49beb8804e..5ff14c7ec753 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -3003,7 +3003,6 @@ static const struct sys_reg_desc sys_reg_descs[] = {
 					ID_AA64MMFR0_EL1_ASIDBITS)),
 	ID_WRITABLE(ID_AA64MMFR1_EL1, ~(ID_AA64MMFR1_EL1_RES0 |
 					ID_AA64MMFR1_EL1_XNX |
-					ID_AA64MMFR1_EL1_VH |
 					ID_AA64MMFR1_EL1_VMIDBits)),
 	ID_FILTERED(ID_AA64MMFR2_EL1,
 		    id_aa64mmfr2_el1, ~(ID_AA64MMFR2_EL1_RES0 |
diff --git a/tools/testing/selftests/kvm/arm64/set_id_regs.c b/tools/testing/selftests/kvm/arm64/set_id_regs.c
index a889550d8aa4..00dd8066b46f 100644
--- a/tools/testing/selftests/kvm/arm64/set_id_regs.c
+++ b/tools/testing/selftests/kvm/arm64/set_id_regs.c
@@ -172,6 +172,7 @@ static const struct reg_ftr_bits ftr_id_aa64mmfr1_el1[] = {
 	REG_FTR_BITS(FTR_LOWER_SAFE, ID_AA64MMFR1_EL1, PAN, 0),
 	REG_FTR_BITS(FTR_LOWER_SAFE, ID_AA64MMFR1_EL1, LO, 0),
 	REG_FTR_BITS(FTR_LOWER_SAFE, ID_AA64MMFR1_EL1, HPDS, 0),
+	REG_FTR_BITS(FTR_LOWER_SAFE, ID_AA64MMFR1_EL1, VH, 0),
 	REG_FTR_BITS(FTR_LOWER_SAFE, ID_AA64MMFR1_EL1, HAFDBS, 0),
 	REG_FTR_END,
 };
-- 
2.33.0

Hi Jinqian,

On Tue, Sep 09, 2025 at 11:44:15AM +0800, Jinqian Yang wrote:
> Allow userspace to downgrade VH in ID_AA64MMFR1_EL1. Userspace can
> only change this value from high to low.

I'm afraid we can't allow this. When we expose FEAT_VHE to the VM,
HCR_EL2.E2H is RES1. Meaning, vEL2 is unconditionally in a VHE
context.

I would be OK with a clarifying comment documenting why the field is
non-writable.

Thanks,
Oliver

On 2025/9/9 13:32, Oliver Upton wrote:
> Hi Jinqian,
> 
> On Tue, Sep 09, 2025 at 11:44:15AM +0800, Jinqian Yang wrote:
>> Allow userspace to downgrade VH in ID_AA64MMFR1_EL1. Userspace can
>> only change this value from high to low.
> 
> I'm afraid we can't allow this. When we expose FEAT_VHE to the VM,
> HCR_EL2.E2H is RES1. Meaning, vEL2 is unconditionally in a VHE
> context.
> 
> I would be OK with a clarifying comment documenting why the field is
> non-writable.
> 

Okay, v3 will keep ID_AA64MMFR1_EL1.VH non-writable.

Thanks,
Jinqian