target/arm: Don't use a TLB for ARMMMUIdx_Stage2

We define ARMMMUIdx_Stage2 as being an MMU index which uses a QEMU
TLB.  However we never actually use the TLB -- all stage 2 lookups
are done by direct calls to get_phys_addr_lpae() followed by a
physical address load via address_space_ld*().

Remove Stage2 from the list of ARM MMU indexes which correspond to
real core MMU indexes, and instead put it in the set of "NOTLB" ARM
MMU indexes.

This allows us to drop NB_MMU_MODES to 11.  It also means we can
safely add support for the ARMv8.3-TTS2UXN extension, which adds
permission bits to the stage 2 descriptors which define execute
permission separatel for EL0 and EL1; supporting that while keeping
Stage2 in a QEMU TLB would require us to use separate TLBs for
"Stage2 for an EL0 access" and "Stage2 for an EL1 access", which is a
lot of extra complication given we aren't even using the QEMU TLB.

In the process of updating the comment on our MMU index use,
fix a couple of other minor errors:
 * NS EL2 EL2&0 was missing from the list in the comment
 * some text hadn't been updated from when we bumped NB_MMU_MODES
   above 8

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200330210400.11724-2-peter.maydell@linaro.org

1 files changed, 15 insertions, 6 deletions

diff --git a/target/arm/cpu.h b/target/arm/cpu.h
index 8b9f2961ba..fe03a74bf0 100644
--- a/target/arm/cpu.h
+++ b/target/arm/cpu.h
@@ -2801,6 +2801,9 @@ bool write_cpustate_to_list(ARMCPU *cpu, bool kvm_sync);
  *     handling via the TLB. The only way to do a stage 1 translation without
  *     the immediate stage 2 translation is via the ATS or AT system insns,
  *     which can be slow-pathed and always do a page table walk.
+ *     The only use of stage 2 translations is either as part of an s1+2
+ *     lookup or when loading the descriptors during a stage 1 page table walk,
+ *     and in both those cases we don't use the TLB.
  *  4. we can also safely fold together the "32 bit EL3" and "64 bit EL3"
  *     translation regimes, because they map reasonably well to each other
  *     and they can't both be active at the same time.
@@ -2816,15 +2819,15 @@ bool write_cpustate_to_list(ARMCPU *cpu, bool kvm_sync);
  * NS EL1 EL1&0 stage 1+2 (aka NS PL1)
  * NS EL1 EL1&0 stage 1+2 +PAN
  * NS EL0 EL2&0
+ * NS EL2 EL2&0
  * NS EL2 EL2&0 +PAN
  * NS EL2 (aka NS PL2)
  * S EL0 EL1&0 (aka S PL0)
  * S EL1 EL1&0 (not used if EL3 is 32 bit)
  * S EL1 EL1&0 +PAN
  * S EL3 (aka S PL1)
- * NS EL1&0 stage 2
  *
- * for a total of 12 different mmu_idx.
+ * for a total of 11 different mmu_idx.
  *
  * R profile CPUs have an MPU, but can use the same set of MMU indexes
  * as A profile. They only need to distinguish NS EL0 and NS EL1 (and
@@ -2846,7 +2849,8 @@ bool write_cpustate_to_list(ARMCPU *cpu, bool kvm_sync);
  * are not quite the same -- different CPU types (most notably M profile
  * vs A/R profile) would like to use MMU indexes with different semantics,
  * but since we don't ever need to use all of those in a single CPU we
- * can avoid setting NB_MMU_MODES to more than 8. The lower bits of
+ * can avoid having to set NB_MMU_MODES to "total number of A profile MMU
+ * modes + total number of M profile MMU modes". The lower bits of
  * ARMMMUIdx are the core TLB mmu index, and the higher bits are always
  * the same for any particular CPU.
  * Variables of type ARMMUIdx are always full values, and the core
@@ -2894,8 +2898,6 @@ typedef enum ARMMMUIdx {
     ARMMMUIdx_SE10_1_PAN = 9 | ARM_MMU_IDX_A,
     ARMMMUIdx_SE3        = 10 | ARM_MMU_IDX_A,
 
-    ARMMMUIdx_Stage2     = 11 | ARM_MMU_IDX_A,
-
     /*
      * These are not allocated TLBs and are used only for AT system
      * instructions or for the first stage of an S12 page table walk.
@@ -2903,6 +2905,14 @@ typedef enum ARMMMUIdx {
     ARMMMUIdx_Stage1_E0 = 0 | ARM_MMU_IDX_NOTLB,
     ARMMMUIdx_Stage1_E1 = 1 | ARM_MMU_IDX_NOTLB,
     ARMMMUIdx_Stage1_E1_PAN = 2 | ARM_MMU_IDX_NOTLB,
+    /*
+     * Not allocated a TLB: used only for second stage of an S12 page
+     * table walk, or for descriptor loads during first stage of an S1
+     * page table walk. Note that if we ever want to have a TLB for this
+     * then various TLB flush insns which currently are no-ops or flush
+     * only stage 1 MMU indexes will need to change to flush stage 2.
+     */
+    ARMMMUIdx_Stage2     = 3 | ARM_MMU_IDX_NOTLB,
 
     /*
      * M-profile.
@@ -2936,7 +2946,6 @@ typedef enum ARMMMUIdxBit {
     TO_CORE_BIT(SE10_1),
     TO_CORE_BIT(SE10_1_PAN),
     TO_CORE_BIT(SE3),
-    TO_CORE_BIT(Stage2),
 
     TO_CORE_BIT(MUser),
     TO_CORE_BIT(MPriv),