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Re: [Qemu-ppc] [PATCH v3] tcg/ppc: Improve unaligned load/store handling


From: Richard Henderson
Subject: Re: [Qemu-ppc] [PATCH v3] tcg/ppc: Improve unaligned load/store handling on 64-bit backend
Date: Tue, 21 Jul 2015 07:27:22 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.1.0

On 07/21/2015 06:19 AM, Benjamin Herrenschmidt wrote:
+    /* Clear the non-page, non-alignment bits from the address */
      if (TCG_TARGET_REG_BITS == 32 || TARGET_LONG_BITS == 32) {
+        /* We don't support unaligned accesses on 32-bits, preserve
+         * the bottom bits and thus trigger a comparison failure on
+         * unaligned accesses
+         */
          tcg_out_rlw(s, RLWINM, TCG_REG_R0, addrlo, 0,
                      (32 - s_bits) & 31, 31 - TARGET_PAGE_BITS);

Why don't you support this unaligned acess with 32-bit guests?

-    } else if (!s_bits) {
-        tcg_out_rld(s, RLDICR, TCG_REG_R0, addrlo,
-                    0, 63 - TARGET_PAGE_BITS);
+    } else if (s_bits) {
+        /* > byte access, we need to handle alignment */
+        if ((opc & MO_AMASK) == MO_ALIGN) {
+            /* Alignment required by the front-end, same as 32-bits */
+            tcg_out_rld(s, RLDICL, TCG_REG_R0, addrlo,
+                        64 - TARGET_PAGE_BITS, TARGET_PAGE_BITS - s_bits);
+            tcg_out_rld(s, RLDICL, TCG_REG_R0, TCG_REG_R0, TARGET_PAGE_BITS, 
0);
+       } else {
+           /* We support unaligned accesses, we need to make sure we fail
+            * if we cross a page boundary. The trick is to add the
+            * access_size-1 to the address before masking the low bits.
+            * That will make the address overflow to the next page if we
+            * cross a page boundary which will then force a mismatch of
+            * the TLB compare since the next page cannot possibly be in
+            * the same TLB index.
+            */
+            tcg_out32(s, ADDI | TAI(TCG_REG_R0, addrlo, (1 << s_bits) - 1));
+            tcg_out_rld(s, RLDICR, TCG_REG_R0, TCG_REG_R0,
+                        0, 63 - TARGET_PAGE_BITS);
+        }
      } else {
-        tcg_out_rld(s, RLDICL, TCG_REG_R0, addrlo,
-                    64 - TARGET_PAGE_BITS, TARGET_PAGE_BITS - s_bits);
-        tcg_out_rld(s, RLDICL, TCG_REG_R0, TCG_REG_R0, TARGET_PAGE_BITS, 0);
+        /* Byte access, just chop off the bits below the page index */
+        tcg_out_rld(s, RLDICR, TCG_REG_R0, addrlo, 0, 63 - TARGET_PAGE_BITS);




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