Re: [Qemu-devel] subpage_write() and duplicated memory_region_ops_write tracepoints

[Hollis Blanchard <hollis_blanchard@mentor.com>]

On 12/09/2015 01:12 PM, Paolo Bonzini wrote:
>
> On 09/12/2015 21:54, Hollis Blanchard wrote:
>> #0  trace_memory_region_ops_write (mr=0x185b620, addr=16, absaddr=738205712, value=136, size=4)
>>      at /scratch1/hblancha/install/customq/qemu-2.4.0/src/trace/generated-tracers.h:7374
>> #1  0x000000000045eb8a in *memory_region_write_with_attrs_accessor* (mr=0x185b620, addr=16,
>>      value=0x45203338, size=4, shift=0, mask=4294967295, attrs=...)
>>      at /scratch1/hblancha/install/customq/qemu-2.4.0/src/memory.c:513
>> #2  0x000000000045ed08 in access_with_adjusted_size (addr=16, value=0x45203338, size=4,
>>      access_size_min=1, access_size_max=4, access=0x45eb15 <memory_region_write_with_attrs_accessor>,
>>      mr=0x185b620, attrs=...) at /scratch1/hblancha/install/customq/qemu-2.4.0/src/memory.c:556
>> #3  0x0000000000461ed7 in memory_region_dispatch_write (mr=0x185b620, addr=16, data=136, size=4,
>>      attrs=...) at /scratch1/hblancha/install/customq/qemu-2.4.0/src/memory.c:1214
>> #4  0x0000000000411bbf in address_space_rw (as=0x11f3440, addr=738205712, attrs=...,
>>      buf=0x45203490 "\210", len=4, is_write=true)
>>      at /scratch1/hblancha/install/customq/qemu-2.4.0/src/exec.c:2497
>> #5  0x0000000000411ea9 in address_space_write (as=0x11f3440, addr=738205712, attrs=...,
>>      buf=0x45203490 "\210", len=4) at /scratch1/hblancha/install/customq/qemu-2.4.0/src/exec.c:2579
>> #6  0x0000000000410d89 in subpage_write (opaque=0x19148f0, addr=16, value=136, len=4, attrs=...)
>>      at /scratch1/hblancha/install/customq/qemu-2.4.0/src/exec.c:2139
>> #7  0x000000000045ebb2 in *memory_region_write_with_attrs_accessor* (mr=0x19148f0, addr=16,
>>      value=0x452035a8, size=4, shift=0, mask=4294967295, attrs=...)
>>      at /scratch1/hblancha/install/customq/qemu-2.4.0/src/memory.c:516
>> #8  0x000000000045ed08 in access_with_adjusted_size (addr=16, value=0x452035a8, size=4,
>>      access_size_min=1, access_size_max=8, access=0x45eb15 <memory_region_write_with_attrs_accessor>,
>>      mr=0x19148f0, attrs=...) at /scratch1/hblancha/install/customq/qemu-2.4.0/src/memory.c:556
>> #9  0x0000000000461ed7 in memory_region_dispatch_write (mr=0x19148f0, addr=16, data=136, size=4,
>>      attrs=...) at /scratch1/hblancha/install/customq/qemu-2.4.0/src/memory.c:1214
>> #10 0x000000000046c61c in io_writel (env=0x2aabace89268, iotlbentry=0x2aabace99808, val=136,
>>      addr=18446743523953745936, retaddr=1107508028)
>>      at /scratch1/hblancha/install/customq/qemu-2.4.0/src/softmmu_template.h:470
>> #11 0x000000000046c3cb in helper_le_stl_mmu (env=0x2aabace89268, addr=18446743523953745936, val=136,
>>      oi=33, retaddr=1107508028)
>>      at /scratch1/hblancha/install/customq/qemu-2.4.0/src/softmmu_template.h:510
>> #12 0x0000000042033b3e in code_gen_buffer ()
>>
>>
>> The first tracepoint in each pair is an artifact, and should be omitted.
>> Any suggestions? We could special case "if (mr->ops->write !=
>> subpage_write) { emit tracepoint }", but that's a bit of a hack... :-)
> You can add an argument to the tracepoint for mr->subpage.  I would keep
> the duplicate however.
OK, I understand your suggestion now, but not your rationale. Are you 
thinking about QEMU developers needing to debug the subpage 
implementation? If so, I think tracing guest MMIO activity is a far more 
common need... perhaps we could use a different tracepoint for the latter?

if (mr->subpage)
     trace_memory_region_ops_write_subpage(mr, addr, tmp, size);
else
     trace_memory_region_ops_write(mr, addr, tmp, size);

That way, people debugging guest behavior can enable just the 
memory_region_ops_write tracepoint, and QEMU developers can also enable 
the other.

Hollis Blanchard
Mentor Graphics Emulation Division