hv_exc: Handle M1RACLES mitigation here due to 12.0 spam

macOS 12.0 added a silly ineffective "mitigation" that clears the
register on every context switch. This doesn't actually mitigate
anything, but it does make this sysreg trap performance-critical,
so we have to move its handling into the C side.

Signed-off-by: Hector Martin <marcan@marcan.st>

proxyclient/m1n1/hv.py

@@ -523,9 +523,7 @@ class HV(Reloadable):
 
         name = sysreg_name(enc)
 
-        skip = set((
-            (3, 5, 15, 10, 1), # M1RACLES mitigation
-        ))
+        skip = set()
         shadow = {
             #SPRR_CONFIG_EL1,
             #SPRR_PERM_EL0,
@@ -557,7 +555,8 @@ class HV(Reloadable):
                 if iss.Rt != 31:
                     ctx.regs[iss.Rt] = 0
             else:
-                value = ctx.regs[iss.Rt]
+                if iss.Rt != 31:
+                    value = ctx.regs[iss.Rt]
                 print(f"Skip: msr {name}, x{iss.Rt} = {value:x}")
         else:
             if iss.DIR == MSR_DIR.READ:

src/hv_exc.c

@@ -188,6 +188,11 @@ static bool hv_handle_msr(u64 *regs, u64 iss)
             }
             return true;
 #endif
+        /* M1RACLES reg, handle here due to silly 12.0 "mitigation" */
+        case SYSREG_ISS(sys_reg(3, 5, 15, 10, 1)):
+            if (is_read)
+                regs[rt] = 0;
+            return true;
     }
 
     return false;