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  • Anton Anton anjo
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    • Antony Pavlov's avatar
      qemu-log: add log category for MMU info · 339aaf5b
      Antony Pavlov authored 
      
Running barebox on qemu-system-mips* with '-d unimp' overloads
stderr by very very many mips_cpu_handle_mmu_fault() messages:

  mips_cpu_handle_mmu_fault address=b80003fd ret 0 physical 00000000180003fd prot 3
  mips_cpu_handle_mmu_fault address=a0800884 ret 0 physical 0000000000800884 prot 3
  mips_cpu_handle_mmu_fault pc a080cd80 ad b80003fd rw 0 mmu_idx 0

So it's very difficult to find LOG_UNIMP message.

The mips_cpu_handle_mmu_fault() messages appear on enabling ANY
logging! It's not very handy.

Adding separate log category for *_cpu_handle_mmu_fault()
logging fixes the problem.

Signed-off-by: default avatarAntony Pavlov <antonynpavlov@gmail.com>
Acked-by: default avatarAlexander Graf <agraf@suse.de>
Reviewed-by: default avatarRichard Henderson <rth@twiddle.net>
Message-id: 1418489298-1184-1-git-send-email-antonynpavlov@gmail.com
Signed-off-by: default avatarPeter Maydell <peter.maydell@linaro.org>
      339aaf5b
  21. Sep 01, 2014
    • Xin Tong's avatar
      implementing victim TLB for QEMU system emulated TLB · 88e89a57
      Xin Tong authored 
      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: default avatarXin Tong <trent.tong@gmail.com>
Message-id: 1407202523-23553-1-git-send-email-trent.tong@gmail.com
Reviewed-by: default avatarPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: default avatarPeter Maydell <peter.maydell@linaro.org>
      88e89a57
  23. Jun 05, 2014
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