Re: Armv7 panic on -current, rpi2 buildworld

> On 20 Feb 2023, at 13:15, Kornel Dulęba <[email protected]> wrote:
> 
>> Can you try with 24abb6b82102eec577eff9bd8dd7726e8cab89f4? There were conditional branch instructions that may mean the function to save the VFP state was not being run.
> I'm currently debugging this and applying 24abb6b82102eec577eff9bd8dd7726e8cab89f4 didn't quite help.
> (I've tested it with dbl_and_ull_via_async that Mark shared in another thread.)
> The root cause is located in vfp_save_state. It's called during the dump, right before the assert is triggered:
> 
> 359         /*
> 360          * savectx() will be called on panic with dumppcb as an argument,
> 361          * dumppcb doesn't have pcb_vfpsaved set, so set it to save
> 362          * the VFP registers.
> 363          */
> 364         if (pcb->pcb_vfpsaved == NULL)
> 365                 pcb->pcb_vfpsaved = &pcb->pcb_vfpstate;
> 
> Here pcb_vfpsaved == NULL, since the VFP has never been used in the kernel.
> This triggers the KASSERT in get_vfpcontext, causing the panic.
> Note that arm64 has very similar logic, so I wonder if a similar panic could be observed there.
> Any thoughts?
> 

It looks like cpu_copy_thread is missing setting pcb_vfpsaved so is likely to be invalid.

On arm64 we set the needed data in vfp_new_thread. This was added to simplify adding SVE support, but could be reused on arm if it’s useful.

Andrew