[PATCH v3 1/5] KVM: arm64: selftests: Disable unused TTBR1_EL1 translations

Fuad Tabba
2026-01-06T09:24:28Z

KVM selftests map all guest code and data into the lower virtual address
range (0x0000...) managed by TTBR0_EL1. The upper range (0xFFFF...)
managed by TTBR1_EL1 is unused and uninitialized.

If a guest accesses the upper range, the MMU attempts a translation
table walk using uninitialized registers, leading to unpredictable
behavior.

Set `TCR_EL1.EPD1` to disable translation table walks for TTBR1_EL1,
ensuring that any access to the upper range generates an immediate
Translation Fault. Additionally, set `TCR_EL1.TBI1` (Top Byte Ignore) to
ensure that tagged pointers in the upper range also deterministically
trigger a Translation Fault via EPD1.

Define `TCR_EPD1_MASK`, `TCR_EPD1_SHIFT`, and `TCR_TBI1` in
`processor.h` to support this configuration. These are based on their
definitions in `arch/arm64/include/asm/pgtable-hwdef.h`.

Suggested-by: Will Deacon <will@kernel.org>
Reviewed-by: Itaru Kitayama <itaru.kitayama@fujitsu.com>
Signed-off-by: Fuad Tabba <tabba@google.com>
---
tools/testing/selftests/kvm/include/arm64/processor.h | 4 ++++
tools/testing/selftests/kvm/lib/arm64/processor.c | 2 ++
2 files changed, 6 insertions(+)

diff --git a/tools/testing/selftests/kvm/include/arm64/processor.h b/tools/testing/selftests/kvm/include/arm64/processor.h
index ff928716574d..ac97a1c436fc 100644
--- a/tools/testing/selftests/kvm/include/arm64/processor.h
+++ b/tools/testing/selftests/kvm/include/arm64/processor.h
@@ -90,6 +90,9 @@
#define TCR_TG0_64K (UL(1) << TCR_TG0_SHIFT)
#define TCR_TG0_16K (UL(2) << TCR_TG0_SHIFT)

+#define TCR_EPD1_SHIFT 23
+#define TCR_EPD1_MASK (UL(1) << TCR_EPD1_SHIFT)
+
#define TCR_IPS_SHIFT 32
#define TCR_IPS_MASK (UL(7) << TCR_IPS_SHIFT)
#define TCR_IPS_52_BITS (UL(6) << TCR_IPS_SHIFT)
@@ -97,6 +100,7 @@
#define TCR_IPS_40_BITS (UL(2) << TCR_IPS_SHIFT)
#define TCR_IPS_36_BITS (UL(1) << TCR_IPS_SHIFT)

+#define TCR_TBI1 (UL(1) << 38)
#define TCR_HA (UL(1) << 39)
#define TCR_DS (UL(1) << 59)

diff --git a/tools/testing/selftests/kvm/lib/arm64/processor.c b/tools/testing/selftests/kvm/lib/arm64/processor.c
index d46e4b13b92c..5b379da8cb90 100644
--- a/tools/testing/selftests/kvm/lib/arm64/processor.c
+++ b/tools/testing/selftests/kvm/lib/arm64/processor.c
@@ -384,6 +384,8 @@ void aarch64_vcpu_setup(struct kvm_vcpu *vcpu, struct kvm_vcpu_init *init)

tcr_el1 |= TCR_IRGN0_WBWA | TCR_ORGN0_WBWA | TCR_SH0_INNER;
tcr_el1 |= TCR_T0SZ(vm->va_bits);
+ tcr_el1 |= TCR_TBI1;
+ tcr_el1 |= TCR_EPD1_MASK;
if (use_lpa2_pte_format(vm))
tcr_el1 |= TCR_DS;

--
2.52.0.351.gbe84eed79e-goog