Re: [Qemu-devel] [RFC 1/8] cputlb: add physical address to CPUTLBEntry

[Emilio G. Cota]

On Fri, May 08, 2015 at 14:51:58 -0700, Richard Henderson wrote:
> Ouch.  24 of 64 wasted bytes for 64-bit?
> 
> I wonder if there's a better way we can encode this to avoid 3 copies of the
> virtual address for read/write/code.  Or if we're better off using more than
> one insn to multiply by a non-power-of-two.

Adding one more instruction works well. Perf tests
(# time ./selftest.sh for [1]) show no appreciable difference.

Patch (i386-only) appended.

[1] http://wiki.qemu.org/download/ppc-virtexml507-linux-2_6_34.tgz

> Or if the hardware multiplier is
> fast enough just multiply by the proper constant.

This option should be slower (3-cycle latency for IMUL
on Ivy Bridge/Haswell, probably slower on others),
but would make the code very simple.
Unfortunately I couldn't write a patch to do it due to
my poor grasp of TCG backend code. If someone provides a
patch I'd be glad to test it.

If the appended ends up being the preferred option I
can extend it to support all the TCG targets. I cannot
however test all of them--only have access to x86 and ppc
hardware at the moment.

Thanks,

                Emilio

[PATCH] i386-only: remove sizeof(CPUTLBEntry)=pow2 constraint

Breaks all non-i386 TCG backends! Do not apply.

Signed-off-by: Emilio G. Cota <address@hidden>
---
 include/exec/cpu-defs.h | 23 +++++++++++++++++------
 tcg/i386/tcg-target.c   | 12 +++++++-----
 2 files changed, 24 insertions(+), 11 deletions(-)

diff --git a/include/exec/cpu-defs.h b/include/exec/cpu-defs.h
index 8891f16..716052a 100644
--- a/include/exec/cpu-defs.h
+++ b/include/exec/cpu-defs.h
@@ -96,14 +96,25 @@ typedef struct CPUTLBEntry {
     /* Addend to virtual address to get host address.  IO accesses
        use the corresponding iotlb value.  */
     uintptr_t addend;
-    /* padding to get a power of two size */
-    uint8_t dummy[(1 << CPU_TLB_ENTRY_BITS) -
-                  (sizeof(target_ulong) * 4 +
-                   ((-sizeof(target_ulong) * 4) & (sizeof(uintptr_t) - 1)) +
-                   sizeof(uintptr_t))];
 } CPUTLBEntry;
 
-QEMU_BUILD_BUG_ON(sizeof(CPUTLBEntry) != (1 << CPU_TLB_ENTRY_BITS));
+/*
+ * Fast TLB hits are essential for softmmu performance. Since we hand-code in
+ * assembly the TCG check for TLB hits, we define here a pair of constants that
+ * allow us to use only shifts to obtain a TLBEntry address given its index.
+ * NOTE: The constants below should thus be updated every time changes are
+ * made to the CPUTLBEntry struct. See the compile-time consistency check 
below.
+ */
+#if TARGET_LONG_BITS == 64
+#define CPU_TLB_ENTRY_OUT_SHIFT 3
+#define CPU_TLB_ENTRY_IN_SHIFT  2
+#else
+#define CPU_TLB_ENTRY_OUT_SHIFT (HOST_LONG_BITS == 32 ? 2 : 3)
+#define CPU_TLB_ENTRY_IN_SHIFT  (HOST_LONG_BITS == 32 ? 2 : 1)
+#endif
+
+QEMU_BUILD_BUG_ON(sizeof(CPUTLBEntry) != \
+                (1 + BIT(CPU_TLB_ENTRY_IN_SHIFT)) << CPU_TLB_ENTRY_OUT_SHIFT);
 
 /* The IOTLB is not accessed directly inline by generated TCG code,
  * so the CPUIOTLBEntry layout is not as critical as that of the
diff --git a/tcg/i386/tcg-target.c b/tcg/i386/tcg-target.c
index ab63823..54250f5 100644
--- a/tcg/i386/tcg-target.c
+++ b/tcg/i386/tcg-target.c
@@ -1195,15 +1195,17 @@ static inline void tcg_out_tlb_load(TCGContext *s, 
TCGReg addrlo, TCGReg addrhi,
     tcg_out_mov(s, htype, r0, addrlo);
     tcg_out_mov(s, ttype, r1, addrlo);
 
-    tcg_out_shifti(s, SHIFT_SHR + hrexw, r0,
-                   TARGET_PAGE_BITS - CPU_TLB_ENTRY_BITS);
+    tcg_out_shifti(s, SHIFT_SHR + hrexw, r0, TARGET_PAGE_BITS);
 
     tgen_arithi(s, ARITH_AND + trexw, r1,
                 TARGET_PAGE_MASK | ((1 << s_bits) - 1), 0);
-    tgen_arithi(s, ARITH_AND + hrexw, r0,
-                (CPU_TLB_SIZE - 1) << CPU_TLB_ENTRY_BITS, 0);
+    tgen_arithi(s, ARITH_AND + hrexw, r0, CPU_TLB_SIZE - 1, 0);
 
-    tcg_out_modrm_sib_offset(s, OPC_LEA + hrexw, r0, TCG_AREG0, r0, 0,
+    tcg_out_modrm_sib_offset(s, OPC_LEA + hrexw, r0, r0, r0,
+                             CPU_TLB_ENTRY_IN_SHIFT, 0);
+
+    tcg_out_modrm_sib_offset(s, OPC_LEA + hrexw, r0, TCG_AREG0, r0,
+                             CPU_TLB_ENTRY_OUT_SHIFT,
                              offsetof(CPUArchState, tlb_table[mem_index][0])
                              + which);
 
-- 
1.8.3