Bochs icache perf when writing memory

Hello,

I was recently doing some profiling of Bochs and noticed a lot of time in my build spent in the instruction decoder caused by icache misses. After reading the code intimately for a few days I *think* I have a pretty decent understanding of how it works and may have fixed some performance issues, but I would like to present my changes and see if what I changed is "legal".

In memory.cc I noticed that I was hitting the memcpy-type case in the BX_MEM_C::{read,write}PhysicalPage methods and in every case I documented it was a read/write of 16 bytes. For writes this seemed particularly bad because it would call `pageWriteStampTable.decWriteStamp(a20addr);`, which if my understanding of the icache is correct may cause an entire page to be flushed from the icache for even small writes. I "fixed" this (it still has unnecessary work when the length is not a multiple of 8) and a similar condition in `BX_CPU_C::v2h_write_byte`.

Anecdotally I noticed a very good performance increase by just eyeballing the IPS counter in the bottom-left of the GUI and the number of calls to `bxICache_c::handleSMC` with an 0xFFFF_FFFF mask dropped from 16,125,122 for ~1BN instructions to just 34. Again if my understanding of the icache is correct (and please explain it to me if my understanding is fundamentally broken), this resulted in a substantially fewer whole pages being flushed from the icache if they were previously mapped in. I also did not observe any *visible* negative impacts of this patch and my snapshot appeared to operate normally.

Thanks!

Here is my patch:

diff --git a/bochs_src/cpu/access.cc b/bochs_src/cpu/access.cc
index da1148ad..4a72f5ce 100644
--- a/bochs_src/cpu/access.cc
+++ b/bochs_src/cpu/access.cc
@@ -513,7 +513,7 @@ BX_CPU_C::v2h_write_byte(bx_address laddr, bx_bool user)
       bx_hostpageaddr_t hostPageAddr = tlbEntry->hostPageAddr;
       Bit32u pageOffset = PAGE_OFFSET(laddr);
       Bit8u *hostAddr = (Bit8u*) (hostPageAddr | pageOffset);
-      pageWriteStampTable.decWriteStamp(tlbEntry->ppf);
+      pageWriteStampTable.decWriteStamp(tlbEntry->ppf, 1);
       return hostAddr;
     }
   }
diff --git a/bochs_src/memory/memory.cc b/bochs_src/memory/memory.cc
index e896f1d1..d3fa737c 100644
--- a/bochs_src/memory/memory.cc
+++ b/bochs_src/memory/memory.cc
@@ -110,8 +110,6 @@ mem_write:
       // len == other, just fall thru to special cases handling
     }
 
-    pageWriteStampTable.decWriteStamp(a20addr);
-
 #ifdef BX_LITTLE_ENDIAN
     data_ptr = (Bit8u *) data;
 #else // BX_BIG_ENDIAN
@@ -122,18 +120,38 @@ mem_write:
     {
       // addr *not* in range 000A0000 .. 000FFFFF
       while(1) {
-        *(BX_MEM_THIS get_vector(a20addr)) = *data_ptr;
-        if (len == 1) return;
-        len--;
-        a20addr++;
-#ifdef BX_LITTLE_ENDIAN
-        data_ptr++;
-#else // BX_BIG_ENDIAN
-        data_ptr--;
-#endif
+        // Read in chunks of 8 bytes if we can
+        if (len % 8 == 0) {
+          pageWriteStampTable.decWriteStamp(a20addr, 8);
+          WriteHostQWordToLittleEndian(BX_MEM_THIS get_vector(a20addr), *(Bit64u*)data_ptr);
+          len -= 8;
+          a20addr += 8;
+          #ifdef BX_LITTLE_ENDIAN
+            data_ptr += 8;
+          #else
+            data_ptr -= 8;
+          #endif
+
+          if (len == 0) return;
+        } else {
+          pageWriteStampTable.decWriteStamp(a20addr, 1);
+          *(BX_MEM_THIS get_vector(a20addr)) = *data_ptr;
+          if (len == 1) return;
+          len--;
+          a20addr++;
+  #ifdef BX_LITTLE_ENDIAN
+          data_ptr++;
+  #else // BX_BIG_ENDIAN
+          data_ptr--;
+  #endif
+        }
       }
     }
 
+
+    printf("LENGTH NOT HANDLED!!! %d\n", len);
+    pageWriteStampTable.decWriteStamp(a20addr);
+
     // addr must be in range 000A0000 .. 000FFFFF
 
     for(unsigned i=0; i<len; i++) {
@@ -273,15 +291,29 @@ mem_read:
     {
       // addr *not* in range 000A0000 .. 000FFFFF
       while(1) {
-        *data_ptr = *(BX_MEM_THIS get_vector(a20addr));
-        if (len == 1) return;
-        len--;
-        a20addr++;
-#ifdef BX_LITTLE_ENDIAN
-        data_ptr++;
-#else // BX_BIG_ENDIAN
-        data_ptr--;
-#endif
+        // Read in chunks of 8 bytes if we can
+        if (len % 8 == 0) {
+          ReadHostQWordFromLittleEndian(BX_MEM_THIS get_vector(a20addr), *(Bit64u*)data_ptr);
+          len -= 8;
+          a20addr += 8;
+          #ifdef BX_LITTLE_ENDIAN
+            data_ptr += 8;
+          #else
+            data_ptr -= 8;
+          #endif
+
+          if (len == 0) return;
+        } else {
+          *data_ptr = *(BX_MEM_THIS get_vector(a20addr));
+          if (len == 1) return;
+          len--;
+          a20addr++;
+  #ifdef BX_LITTLE_ENDIAN
+          data_ptr++;
+  #else // BX_BIG_ENDIAN
+          data_ptr--;
+  #endif
+        }
       }
     }