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[Qemu-devel] [Bug 1682093] Re: aarch64-softmmu "bad ram pointer" crash

From: Harry Wagstaff
Subject: [Qemu-devel] [Bug 1682093] Re: aarch64-softmmu "bad ram pointer" crash
Date: Wed, 12 Apr 2017 15:02:21 -0000

I've done some investigation and it appears that this bug is caused by
the following:

1. The flash memory of the virt platform is initialised as a
cfi.pflash01. It has a memory region with romd_mode = true and
rom_device = true

2. Some code stored in the flash memory is executed. This causes the
memory to be loaded into the TLB.

3. The code is overwritten. This causes the romd_mode of the flash
memory to be reset. It also causes the code to be evicted from the TLB.

4. An attempt is made to execute the code again (cpu_exec(), cpu-exec.c:677)
4a. Eventually, QEMU attempts to refill the TLB (softmmu_template.h:127)
4b. We try to fill in the tlb entry (tlb_set_page_with_attrs, cputlb.c:602)
4b. The flash memory no longer appears to be a ram or romd (cputlb.c:632)
4c. QEMU decides that the flash memory is an IO device (cputlb.c:634)
4d. QEMU aborts while trying to fill in the rest of the TLB entry 

I have built a MWE (which I have attached) which produces this behaviour
in git head. I'm not exactly sure what a fix for this should look like:
AFAIK it's not technically valid to write into flash, but I'm not sure
that QEMU crashing should be considered correct behaviour.

** Attachment added: "mwe.tar.gz"

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  aarch64-softmmu "bad ram pointer" crash

Status in QEMU:

Bug description:
  I am developing a piece of software called SimBench which is a
  benchmarking system for full system simulators. I am currently porting
  this to aarch64, using QEMU as a test platform.

  I have encountered a 'bad ram pointer' crash. I've attempted to build
  a minimum test case, but I haven't managed to replicate the behaviour
  in isolation, so I've created a branch of my project which exhibits
  the crash: https://bitbucket.org/Awesomeclaw/simbench/get/qemu-

  The package can be compiled using:


  and then run using:

  qemu-system-aarch64  -M virt -m 512 -cpu cortex-a57 -kernel
  out/armv8/virt/simbench -nographic

  I have replicated the issue in both qemu 2.8.1 and in 2.9.0-rc3, on
  Fedora 23. Please let me know if you need any more information or any
  logs/core dumps/etc.

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