Fwd: disabling NMI rendering breakpoints non-functional?

Fwd: disabling NMI rendering breakpoints non-functional?

[Kent Tong]

Hi,

It seems that after the code being traced disables NMI, gdb will not
stop at the next instruction (si) or at breakpoints. Is this a bug or
am I missing something?

The code is seabios running in qemu i386.

Thanks!

--
Kent Tong
IT author and consultant, child education coach

Re: Fwd: disabling NMI rendering breakpoints non-functional?

[Pedro Alves]

On 04/12/2013 04:44 PM, Kent Tong wrote:
> Hi,
> 
> It seems that after the code being traced disables NMI, gdb will not
> stop at the next instruction (si) or at breakpoints. Is this a bug or
> am I missing something?
> 
> The code is seabios running in qemu i386.

GDB has no clue of NMI, it just tells the target to set a breakpoint at
some address, or to single-step.  How qemu implements those things
behind the scenes is up to qemu.

-- 
Pedro Alves

Re: Fwd: disabling NMI rendering breakpoints non-functional?

[Kent Tong]

> GDB has no clue of NMI, it just tells the target to set a breakpoint at
> some address, or to single-step.  How qemu implements those things
> behind the scenes is up to qemu.

Thanks for the reply! I also figured that. But does the breakpoint if it is
set to a memory address whose content will be overwritten (e.g., loading
instructions into there)? In the past it worked, but now somehow it has
stopped working.

--
Kent Tong
IT author and consultant, child education coach

Re: Fwd: disabling NMI rendering breakpoints non-functional?

[Pedro Alves]

On 04/16/2013 03:09 AM, Kent Tong wrote:
>> GDB has no clue of NMI, it just tells the target to set a breakpoint at
>> some address, or to single-step.  How qemu implements those things
>> behind the scenes is up to qemu.
> 
> Thanks for the reply! I also figured that. But does the breakpoint if it is
> set to a memory address whose content will be overwritten (e.g., loading
> instructions into there)? In the past it worked, but now somehow it has
> stopped working.

Depends on how qemu implements breakpoints. By default, if the target
doesn't support z0 remote protocol packets, then GDB fallsback to implementing
breakpoints by planting architecture-defined breakpoint instructions (on x86
that's trap3/0xcc), and then handling the resulting trap.  Naturally it
won't work if the instruction is overwritten.  The emulator is free to
implement the z0/Z0 packets to handle setting breakpoints itself as it
sees fit.  It could still be by planting 0xcc instructions or by watching
the address that is about to be executed, or through whatever other means
it comes up with to make that scenario work.  GDB doesn't care.

-- 
Pedro Alves

Re: Fwd: disabling NMI rendering breakpoints non-functional?

[Kent Tong]

> ...The emulator is free to
> implement the z0/Z0 packets to handle setting breakpoints itself as it
> sees fit.  It could still be by planting 0xcc instructions or by watching
> the address that is about to be executed, or through whatever other means
> it comes up with to make that scenario work.  GDB doesn't care.

Thanks for the info! I guess that nailed the issue.


--
Kent Tong
IT author and consultant, child education coach