target/arm: Implement FEAT_S2FWB

Implement the handling of FEAT_S2FWB; the meat of this is in the new
combined_attrs_fwb() function which combines S1 and S2 attributes
when HCR_EL2.FWB is set.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220505183950.2781801-4-peter.maydell@linaro.org

target/arm/cpu.h

@@ -3941,6 +3941,11 @@ static inline bool isar_feature_aa64_st(const ARMISARegisters *id)
     return FIELD_EX64(id->id_aa64mmfr2, ID_AA64MMFR2, ST) != 0;
 }
 
+static inline bool isar_feature_aa64_fwb(const ARMISARegisters *id)
+{
+    return FIELD_EX64(id->id_aa64mmfr2, ID_AA64MMFR2, FWB) != 0;
+}
+
 static inline bool isar_feature_aa64_bti(const ARMISARegisters *id)
 {
     return FIELD_EX64(id->id_aa64pfr1, ID_AA64PFR1, BT) != 0;

target/arm/helper.c

@@ -5161,6 +5161,9 @@ static void do_hcr_write(CPUARMState *env, uint64_t value, uint64_t valid_mask)
         if (cpu_isar_feature(aa64_scxtnum, cpu)) {
             valid_mask |= HCR_ENSCXT;
         }
+        if (cpu_isar_feature(aa64_fwb, cpu)) {
+            valid_mask |= HCR_FWB;
+        }
     }
 
     /* Clear RES0 bits.  */
@@ -5172,8 +5175,10 @@ static void do_hcr_write(CPUARMState *env, uint64_t value, uint64_t valid_mask)
      * HCR_PTW forbids certain page-table setups
      * HCR_DC disables stage1 and enables stage2 translation
      * HCR_DCT enables tagging on (disabled) stage1 translation
+     * HCR_FWB changes the interpretation of stage2 descriptor bits
      */
-    if ((env->cp15.hcr_el2 ^ value) & (HCR_VM | HCR_PTW | HCR_DC | HCR_DCT)) {
+    if ((env->cp15.hcr_el2 ^ value) &
+        (HCR_VM | HCR_PTW | HCR_DC | HCR_DCT | HCR_FWB)) {
         tlb_flush(CPU(cpu));
     }
     env->cp15.hcr_el2 = value;
@@ -10731,9 +10736,15 @@ static bool ptw_attrs_are_device(CPUARMState *env, ARMCacheAttrs cacheattrs)
      * attributes are therefore only Device if stage 2 specifies Device.
      * With HCR_EL2.FWB == 0 this is when descriptor bits [5:4] are 0b00,
      * ie when cacheattrs.attrs bits [3:2] are 0b00.
+     * With HCR_EL2.FWB == 1 this is when descriptor bit [4] is 0, ie
+     * when cacheattrs.attrs bit [2] is 0.
      */
     assert(cacheattrs.is_s2_format);
-    return (cacheattrs.attrs & 0xc) == 0;
+    if (arm_hcr_el2_eff(env) & HCR_FWB) {
+        return (cacheattrs.attrs & 0x4) == 0;
+    } else {
+        return (cacheattrs.attrs & 0xc) == 0;
+    }
 }
 
 /* Translate a S1 pagetable walk through S2 if needed.  */
@@ -12618,6 +12629,69 @@ static uint8_t combined_attrs_nofwb(CPUARMState *env,
     return ret_attrs;
 }
 
+static uint8_t force_cacheattr_nibble_wb(uint8_t attr)
+{
+    /*
+     * Given the 4 bits specifying the outer or inner cacheability
+     * in MAIR format, return a value specifying Normal Write-Back,
+     * with the allocation and transient hints taken from the input
+     * if the input specified some kind of cacheable attribute.
+     */
+    if (attr == 0 || attr == 4) {
+        /*
+         * 0 == an UNPREDICTABLE encoding
+         * 4 == Non-cacheable
+         * Either way, force Write-Back RW allocate non-transient
+         */
+        return 0xf;
+    }
+    /* Change WriteThrough to WriteBack, keep allocation and transient hints */
+    return attr | 4;
+}
+
+/*
+ * Combine the memory type and cacheability attributes of
+ * s1 and s2 for the HCR_EL2.FWB == 1 case, returning the
+ * combined attributes in MAIR_EL1 format.
+ */
+static uint8_t combined_attrs_fwb(CPUARMState *env,
+                                  ARMCacheAttrs s1, ARMCacheAttrs s2)
+{
+    switch (s2.attrs) {
+    case 7:
+        /* Use stage 1 attributes */
+        return s1.attrs;
+    case 6:
+        /*
+         * Force Normal Write-Back. Note that if S1 is Normal cacheable
+         * then we take the allocation hints from it; otherwise it is
+         * RW allocate, non-transient.
+         */
+        if ((s1.attrs & 0xf0) == 0) {
+            /* S1 is Device */
+            return 0xff;
+        }
+        /* Need to check the Inner and Outer nibbles separately */
+        return force_cacheattr_nibble_wb(s1.attrs & 0xf) |
+            force_cacheattr_nibble_wb(s1.attrs >> 4) << 4;
+    case 5:
+        /* If S1 attrs are Device, use them; otherwise Normal Non-cacheable */
+        if ((s1.attrs & 0xf0) == 0) {
+            return s1.attrs;
+        }
+        return 0x44;
+    case 0 ... 3:
+        /* Force Device, of subtype specified by S2 */
+        return s2.attrs << 2;
+    default:
+        /*
+         * RESERVED values (including RES0 descriptor bit [5] being nonzero);
+         * arbitrarily force Device.
+         */
+        return 0;
+    }
+}
+
 /* Combine S1 and S2 cacheability/shareability attributes, per D4.5.4
  * and CombineS1S2Desc()
  *
@@ -12652,7 +12726,11 @@ static ARMCacheAttrs combine_cacheattrs(CPUARMState *env,
     }
 
     /* Combine memory type and cacheability attributes */
-    ret.attrs = combined_attrs_nofwb(env, s1, s2);
+    if (arm_hcr_el2_eff(env) & HCR_FWB) {
+        ret.attrs = combined_attrs_fwb(env, s1, s2);
+    } else {
+        ret.attrs = combined_attrs_nofwb(env, s1, s2);
+    }
 
     /*
      * Any location for which the resultant memory type is any