mode processor mode switch

John Baldwin
Fri Jan 17 21:46:00 GMT 2020

On 1/16/20 10:52 AM, Pedro Alves wrote:
> On 1/16/20 2:51 PM, Luis Machado wrote:
>> Hi,
>> On 1/14/20 8:58 AM, Jirka Koutný wrote:
>>> Hello,
>>> I have a 32-bit elf executable which at some point switches to long mode
>>> (kernel is 64-bit). Is there a way to tell gdb about the .code32/64
>>> directives? Because expectedly the switch messes up disassembly and
>>> stepping.
>>> Thank you
>>> Jirka
>> Unfortunately i don't think there is a good way to achieve this with the current implementation.
>> You could teach GDB about the quirks in the architecture, but it sounds better to have a more general solution.
>> I'm working on making this more flexible though, since i have a need to make the architecture information per-thread, at least the target description with the registers and types.
> For x86-64 in particular, I think the ideal solution would be for
> the remote target to always report the widest mode it supports,
> which would be 64-bit, and then do the 32-bit/16-bit modes
> presentation all on the gdb side (i.e., user-visible 32-bit on
> top of 64-bit description).  Mode switching would not change the remote
> target description.  This is unlike the current architecture where a
> remote server reports a 32-bit description for a 32-bit process even
> if the remote server is actually running on a 64-bit machine.

I think this is the way to go, but you might still need some way to specify
the mode.  I'm not quite sure what qemu does, but for a gdb stub I've
worked on for bhyve (a KVM-like hypervisor in FreeBSD), it always reports
the architecture as 64-bits but checks various control registers when
resolving virtual addresses for the 'm' and 'M' protocol commands.

John Baldwin

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