KVM: arm64: selftests: Disable unused TTBR1_EL1 translations

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>
Link: https://patch.msgid.link/20260109082218.3236580-2-tabba@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

Author: Fuad Tabba

tools/testing/selftests/kvm/include/arm64/processor.h

@@ -90,13 +90,17 @@
 #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)
 #define TCR_IPS_48_BITS	(UL(5) << TCR_IPS_SHIFT)
 #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)
 

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;