[PATCH v2] KVM: arm64: selftests: Skip unwritable ID fields in the failed-run test

[Sasha Levin <sashal@kernel.org>]

test_idreg_frozen_after_failed_run() checks that changing a feature ID
register after a failed first KVM_RUN is rejected with -EBUSY, and does
so for every field of every register in test_regs.

On a host without AArch32 support at EL0, KVM treats the AArch32 ID
registers as RAZ/WI: aa32_id_visibility() returns REG_RAZ | REG_USER_WI,
and kvm_sys_reg_set_user() ignores the write and returns 0 before
set_id_reg() checks whether the ID registers are final. The test then
fails on the first such register:

  arm64/set_id_regs.c:870: r < 0 && errno == EBUSY
  ID_DFR0_EL1_PerfMon write after failed first run: r=0 errno=16

KernelCI hits this on every run on an AArch64 only GB300 host.

Only a field that userspace can change can be frozen, and KVM reports
these registers as not writable in KVM_ARM_GET_REG_WRITABLE_MASKS. Skip
the fields that the writable mask does not cover. test_vm_ftr_id_regs()
already requires the mask to cover every other field in test_regs, so
this only drops the fields KVM never lets userspace change. Whether KVM
accepts writes to fields it reports as not writable is a separate
question from finalization.

Fixes: 00e309933879 ("KVM: arm64: selftests: Check ID regs are immutable after a failed run")
Link: https://d.kernelci.org/test/maestro:6adcad0c55fda50760800a00
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
Changes in v2:
- Skip every field that KVM_ARM_GET_REG_WRITABLE_MASKS does not report as
  writable, instead of special casing the AArch32 ID registers (Mark Brown).

v1: https://lore.kernel.org/all/20261024161118.1007472-1-sashal@kernel.org/

 tools/testing/selftests/kvm/arm64/set_id_regs.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/kvm/arm64/set_id_regs.c b/tools/testing/selftests/kvm/arm64/set_id_regs.c
index 10849d21c0dd2..1d2cc25cb3f4c 100644
--- a/tools/testing/selftests/kvm/arm64/set_id_regs.c
+++ b/tools/testing/selftests/kvm/arm64/set_id_regs.c
@@ -815,6 +815,10 @@ static void test_idreg_frozen_after_failed_run(void)
 		SYS_REVIDR_EL1,
 		SYS_AIDR_EL1,
 	};
+	u64 masks[KVM_ARM_FEATURE_ID_RANGE_SIZE];
+	struct reg_mask_range range = {
+		.addr = (__u64)masks,
+	};
 	struct kvm_vcpu_init init;
 	struct kvm_vcpu *vcpu;
 	struct kvm_vm *vm;
@@ -831,6 +835,7 @@ static void test_idreg_frozen_after_failed_run(void)
 
 	vm = vm_create(1);
 	vm_enable_cap(vm, KVM_CAP_ARM_WRITABLE_IMP_ID_REGS, 0);
+	vm_ioctl(vm, KVM_ARM_GET_REG_WRITABLE_MASKS, &range);
 	kvm_get_default_vcpu_target(vm, &init);
 	init.features[0] |= (1 << KVM_ARM_VCPU_PMU_V3);
 	vcpu = aarch64_vcpu_add(vm, 0, &init, guest_code);
@@ -852,7 +857,9 @@ static void test_idreg_frozen_after_failed_run(void)
 	 */
 	for (int i = 0; i < ARRAY_SIZE(test_regs); i++) {
 		const struct reg_ftr_bits *ftr_bits = test_regs[i].ftr_bits;
-		u64 reg = KVM_ARM64_SYS_REG(test_regs[i].reg);
+		u32 reg_id = test_regs[i].reg;
+		u64 reg = KVM_ARM64_SYS_REG(reg_id);
+		u64 mask = masks[encoding_to_range_idx(reg_id)];
 		u64 val = vcpu_get_reg(vcpu, reg);
 
 		for (int j = 0; ftr_bits[j].type != FTR_END; j++) {
@@ -860,6 +867,10 @@ static void test_idreg_frozen_after_failed_run(void)
 			u64 safe = get_safe_value(&ftr_bits[j], ftr);
 			u64 new_val;
 
+			/* Only a field userspace could change can be frozen. */
+			if ((mask & ftr_bits[j].mask) != ftr_bits[j].mask)
+				continue;
+
 			if (safe == ftr)
 				continue;
 

base-commit: 21a7f44fd0765fcc5194130b28767786eba1d1e7
-- 
2.53.0