FEAT_E2H0 is a formalisation of the existing behaviour of HCR_EL2.E2H
being programmable to switch between EL2 host mode and the
"traditional" nVHE EL2 mode. This implies at some point we might want
to model CPUs without FEAT_E2H0 which will always have EL2 host mode
enabled.

There are two values to represent no E2H0 systems of which 0b1110 will
make HCR_EL2.NV1 RES0 for FEAT_NV systems. For FEAT_NV2 the NV1 bit is
always valid.

Message-ID: <20260130181648.628364-1-alex.bennee@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Mohamed Mediouni <mohamed@unpredictable.fr>

---
v3
  - treat E2H0 as a signed field
  - don't duplicate FEAT_NV check
  - fold HCR_E2H into wider FEAT_AARCH64 check along with HCR_RW
  - don't force writable NV1 for FEAT_NV2
v2
  - new helper and properly handling NV1
---
 docs/system/arm/emulation.rst |  1 +
 target/arm/cpu-features.h     | 15 +++++++++++++++
 target/arm/helper.c           | 21 +++++++++++++++------
 3 files changed, 31 insertions(+), 6 deletions(-)

diff --git a/docs/system/arm/emulation.rst b/docs/system/arm/emulation.rst
index e0d5f9886e1..7787691853e 100644
--- a/docs/system/arm/emulation.rst
+++ b/docs/system/arm/emulation.rst
@@ -54,6 +54,7 @@ the following architecture extensions:
 - FEAT_DotProd (Advanced SIMD dot product instructions)
 - FEAT_DoubleFault (Double Fault Extension)
 - FEAT_E0PD (Preventing EL0 access to halves of address maps)
+- FEAT_E2H0 (Programming of HCR_EL2.E2H)
 - FEAT_EBF16 (AArch64 Extended BFloat16 instructions)
 - FEAT_ECV (Enhanced Counter Virtualization)
 - FEAT_EL0 (Support for execution at EL0)
diff --git a/target/arm/cpu-features.h b/target/arm/cpu-features.h
index b29e20eeb68..b683c9551a0 100644
--- a/target/arm/cpu-features.h
+++ b/target/arm/cpu-features.h
@@ -347,6 +347,7 @@ FIELD(ID_AA64MMFR3, ADERR, 56, 4)
 FIELD(ID_AA64MMFR3, SPEC_FPACC, 60, 4)
 
 FIELD(ID_AA64MMFR4, ASID2, 8, 4)
+FIELD(ID_AA64MMFR4, E2H0, 24, 4)
 
 FIELD(ID_AA64DFR0, DEBUGVER, 0, 4)
 FIELD(ID_AA64DFR0, TRACEVER, 4, 4)
@@ -1378,6 +1379,20 @@ static inline bool isar_feature_aa64_asid2(const ARMISARegisters *id)
     return FIELD_EX64_IDREG(id, ID_AA64MMFR4, ASID2) != 0;
 }
 
+/*
+ * Note the E2H0 ID fields is signed, increasingly negative as more
+ * isn't implemented.
+ */
+static inline bool isar_feature_aa64_e2h0(const ARMISARegisters *id)
+{
+    return FIELD_SEX64_IDREG(id, ID_AA64MMFR4, E2H0) >= 0;
+}
+
+static inline bool isar_feature_aa64_nv1_res0(const ARMISARegisters *id)
+{
+    return FIELD_SEX64_IDREG(id, ID_AA64MMFR4, E2H0) <= -2;
+}
+
 static inline bool isar_feature_aa64_mec(const ARMISARegisters *id)
 {
     return FIELD_EX64_IDREG(id, ID_AA64MMFR3, MEC) != 0;
diff --git a/target/arm/helper.c b/target/arm/helper.c
index 390ea32c218..e12b2455d3f 100644
--- a/target/arm/helper.c
+++ b/target/arm/helper.c
@@ -3776,7 +3776,8 @@ static void do_hcr_write(CPUARMState *env, uint64_t value, uint64_t valid_mask)
     }
 
     if (arm_feature(env, ARM_FEATURE_AARCH64)) {
-        if (cpu_isar_feature(aa64_vh, cpu)) {
+        if (cpu_isar_feature(aa64_vh, cpu) &&
+            cpu_isar_feature(aa64_e2h0, cpu)) {
             valid_mask |= HCR_E2H;
         }
         if (cpu_isar_feature(aa64_ras, cpu)) {
@@ -3801,7 +3802,10 @@ static void do_hcr_write(CPUARMState *env, uint64_t value, uint64_t valid_mask)
             valid_mask |= HCR_GPF;
         }
         if (cpu_isar_feature(aa64_nv, cpu)) {
-            valid_mask |= HCR_NV | HCR_NV1 | HCR_AT;
+            valid_mask |= HCR_NV | HCR_AT;
+            if (!cpu_isar_feature(aa64_nv1_res0, cpu)) {
+                valid_mask |= HCR_NV1;
+            }
         }
         if (cpu_isar_feature(aa64_nv2, cpu)) {
             valid_mask |= HCR_NV2;
@@ -3817,10 +3821,15 @@ static void do_hcr_write(CPUARMState *env, uint64_t value, uint64_t valid_mask)
     /* Clear RES0 bits.  */
     value &= valid_mask;
 
-    /* RW is RAO/WI if EL1 is AArch64 only */
-    if (arm_feature(env, ARM_FEATURE_AARCH64) &&
-        !cpu_isar_feature(aa64_aa32_el1, cpu)) {
-        value |= HCR_RW;
+    if (arm_feature(env, ARM_FEATURE_AARCH64)) {
+        /* RW is RAO/WI if EL1 is AArch64 only */
+        if (!cpu_isar_feature(aa64_aa32_el1, cpu)) {
+            value |= HCR_RW;
+        }
+        /* Strictly E2H is RES1 unless FEAT_E2H0 relaxes the requirement */
+        if (!cpu_isar_feature(aa64_e2h0, cpu)) {
+            value |= HCR_E2H;
+        }
     }
 
     /*
-- 
2.47.3