[PATCH] target/arm: Implement (trivially) ARMv8.2-TTCNP

[Peter Maydell]

The ARMv8.2-TTCNP extension allows an implementation to optimize by
sharing TLB entries between multiple cores, provided that software
declares that it's ready to deal with this by setting a CnP bit in
the TTBRn_ELx.  It is mandatory from ARMv8.2 onward.

For QEMU's TLB implementation, sharing TLB entries between different
cores would not really benefit us and would be a lot of work to
implement.  So we implement this extension in the "trivial" manner:
we allow the guest to set and read back the CnP bit, but don't change
our behaviour (this is an architecturally valid implementation
choice).

The only code path which looks at the TTBRn_ELx values for the
long-descriptor format where the CnP bit is defined is already doing
enough masking to not get confused when the CnP bit at the bottom of
the register is set, so we can simply add a comment noting why we're
relying on that mask.

Signed-off-by: Peter Maydell <address@hidden>
---
 target/arm/cpu.c    | 1 +
 target/arm/cpu64.c  | 2 ++
 target/arm/helper.c | 4 ++++
 3 files changed, 7 insertions(+)

diff --git a/target/arm/cpu.c b/target/arm/cpu.c
index 2eadf4dcb8b..64dc9509927 100644
--- a/target/arm/cpu.c
+++ b/target/arm/cpu.c
@@ -2720,6 +2720,7 @@ static void arm_max_initfn(Object *obj)
             t = cpu->isar.id_mmfr4;
             t = FIELD_DP32(t, ID_MMFR4, HPDS, 1); /* AA32HPD */
             t = FIELD_DP32(t, ID_MMFR4, AC2, 1); /* ACTLR2, HACTLR2 */
+            t = FIELD_DP32(t, ID_MMFR4, CNP, 1); /* TTCNP */
             cpu->isar.id_mmfr4 = t;
         }
 #endif
diff --git a/target/arm/cpu64.c b/target/arm/cpu64.c
index 0929401a4dd..e4d793a2415 100644
--- a/target/arm/cpu64.c
+++ b/target/arm/cpu64.c
@@ -679,6 +679,7 @@ static void aarch64_max_initfn(Object *obj)
 
         t = cpu->isar.id_aa64mmfr2;
         t = FIELD_DP64(t, ID_AA64MMFR2, UAO, 1);
+        t = FIELD_DP64(t, ID_AA64MMFR2, CNP, 1); /* TTCNP */
         cpu->isar.id_aa64mmfr2 = t;
 
         /* Replicate the same data to the 32-bit id registers.  */
@@ -705,6 +706,7 @@ static void aarch64_max_initfn(Object *obj)
 
         u = cpu->isar.id_mmfr4;
         u = FIELD_DP32(u, ID_MMFR4, AC2, 1); /* ACTLR2, HACTLR2 */
+        t = FIELD_DP32(t, ID_MMFR4, CNP, 1); /* TTCNP */
         cpu->isar.id_mmfr4 = u;
 
         u = cpu->isar.id_aa64dfr0;
diff --git a/target/arm/helper.c b/target/arm/helper.c
index 79db169e046..911baf7bcb7 100644
--- a/target/arm/helper.c
+++ b/target/arm/helper.c
@@ -10572,6 +10572,10 @@ static bool get_phys_addr_lpae(CPUARMState *env, 
target_ulong address,
 
     /* Now we can extract the actual base address from the TTBR */
     descaddr = extract64(ttbr, 0, 48);
+    /*
+     * We rely on this masking to clear the RES0 bits at the bottom of the TTBR
+     * and also to mask out CnP (bit 0) which could validly be non-zero.
+     */
     descaddr &= ~indexmask;
 
     /* The address field in the descriptor goes up to bit 39 for ARMv7
-- 
2.20.1