implementing victim TLB for QEMU system emulated TLB

QEMU system mode page table walks are expensive. Taken by running QEMU
qemu-system-x86_64 system mode on Intel PIN , a TLB miss and walking a
4-level page tables in guest Linux OS takes ~450 X86 instructions on
average.

QEMU system mode TLB is implemented using a directly-mapped hashtable.
This structure suffers from conflict misses. Increasing the
associativity of the TLB may not be the solution to conflict misses as
all the ways may have to be walked in serial.

A victim TLB is a TLB used to hold translations evicted from the
primary TLB upon replacement. The victim TLB lies between the main TLB
and its refill path. Victim TLB is of greater associativity (fully
associative in this patch). It takes longer to lookup the victim TLB,
but its likely better than a full page table walk. The memory
translation path is changed as follows :

Before Victim TLB:
1. Inline TLB lookup
2. Exit code cache on TLB miss.
3. Check for unaligned, IO accesses
4. TLB refill.
5. Do the memory access.
6. Return to code cache.

After Victim TLB:
1. Inline TLB lookup
2. Exit code cache on TLB miss.
3. Check for unaligned, IO accesses
4. Victim TLB lookup.
5. If victim TLB misses, TLB refill
6. Do the memory access.
7. Return to code cache

The advantage is that victim TLB can offer more associativity to a
directly mapped TLB and thus potentially fewer page table walks while
still keeping the time taken to flush within reasonable limits.
However, placing a victim TLB before the refill path increase TLB
refill path as the victim TLB is consulted before the TLB refill. The
performance results demonstrate that the pros outweigh the cons.

some performance results taken on SPECINT2006 train
datasets and kernel boot and qemu configure script on an
Intel(R) Xeon(R) CPU  E5620  @ 2.40GHz Linux machine are shown in the
Google Doc link below.

https://docs.google.com/spreadsheets/d/1eiItzekZwNQOal_h-5iJmC4tMDi051m9qidi5_nwvH4/edit?usp=sharing

In summary, victim TLB improves the performance of qemu-system-x86_64 by
11% on average on SPECINT2006, kernelboot and qemu configscript and with
highest improvement of in 26% in 456.hmmer. And victim TLB does not result
in any performance degradation in any of the measured benchmarks. Furthermore,
the implemented victim TLB is architecture independent and is expected to
benefit other architectures in QEMU as well.

Although there are measurement fluctuations, the performance
improvement is very significant and by no means in the range of
noises.

Signed-off-by: Xin Tong <trent.tong@gmail.com>
Message-id: 1407202523-23553-1-git-send-email-trent.tong@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

1 files changed, 7 insertions, 2 deletions

diff --git a/include/exec/cpu-defs.h b/include/exec/cpu-defs.h
index 2dd6206d4a..0ca6f0b953 100644
--- a/include/exec/cpu-defs.h
+++ b/include/exec/cpu-defs.h
@@ -71,6 +71,8 @@ typedef uint64_t target_ulong;
 #if !defined(CONFIG_USER_ONLY)
 #define CPU_TLB_BITS 8
 #define CPU_TLB_SIZE (1 << CPU_TLB_BITS)
+/* use a fully associative victim tlb of 8 entries */
+#define CPU_VTLB_SIZE 8
 
 #if HOST_LONG_BITS == 32 && TARGET_LONG_BITS == 32
 #define CPU_TLB_ENTRY_BITS 4
@@ -103,9 +105,12 @@ QEMU_BUILD_BUG_ON(sizeof(CPUTLBEntry) != (1 << CPU_TLB_ENTRY_BITS));
 #define CPU_COMMON_TLB \
     /* The meaning of the MMU modes is defined in the target code. */   \
     CPUTLBEntry tlb_table[NB_MMU_MODES][CPU_TLB_SIZE];                  \
-    hwaddr iotlb[NB_MMU_MODES][CPU_TLB_SIZE];               \
+    CPUTLBEntry tlb_v_table[NB_MMU_MODES][CPU_VTLB_SIZE];               \
+    hwaddr iotlb[NB_MMU_MODES][CPU_TLB_SIZE];                           \
+    hwaddr iotlb_v[NB_MMU_MODES][CPU_VTLB_SIZE];                        \
     target_ulong tlb_flush_addr;                                        \
-    target_ulong tlb_flush_mask;
+    target_ulong tlb_flush_mask;                                        \
+    target_ulong vtlb_index;                                            \
 
 #else