[RFC] Changing gdbarch mid-execution

Hi,

I have a situation at hand and i'm thinking about how to best solve it.

AArch64 SVE has the capability of changing the vector length 
mid-execution. This can be done at the thread level.

Native GDB already supports this by looking at the ptrace data. But 
doing this for a remote target requires changes to the RSP.

Instead of changing things just for this particular case, i'm 
considering having a more general mechanism for updating the 
architecture data whenever such change is noticed by whoever is 
controlling the inferior.

My idea is to get the mechanism started by using the stop reply to send 
a new notification, say, "arch-changed".

That should trigger GDB to re-fetch the architecture data and 
reinitialize it.

In the particular case of SVE, we only need to fetch the target 
description again, so we have the proper vector length and data types set.

Does this sound like a useful feature? Or should i go for the solution 
with less impact that will only take care of re-fetching the target 
description?

Regards,
Luis