Re: [PATCH] arm64: fpsimd: Added API to manage fpsimd state inside kernel

[Dave Martin]

On Fri, Jun 05, 2020 at 11:37:05AM +0100, Mark Rutland wrote:
> Hi Wooyeon,
> 
> There are a *lot* of people Cc' here, many of whomo will find this
> irrelevant. Please try to keep the Cc list constrained to a reasonable
> number of interested parties.
> 
> On Fri, Jun 05, 2020 at 04:30:52PM +0900, Wooyeon Kim wrote:
> > From: Wooki Min <wooki.min@xxxxxxxxxxx>
> > 
> >      This is an patch to use FPSIMD register in Kernel space.
> >      It need to manage to use FPSIMD register without damaging it
> >      of the user task.
> >      Following items have been implemented and added.
> 
> Please introduce the problem you are trying to solve in more detail. We
> already have kernel_neon_{begin,end}() for kernel-mode NEON; why is that
> not sufficient for your needs? Please answer this before considering
> other details.
> 
> What do you want to use this for?
> 
> > 
> >      1. Using FPSIMD in ISR (in_interrupt)
> > 	It can used __efi_fpsimd_begin/__efi_fpsimd_end
> > 	which is already implemented.
> > 	Save fpsimd state before entering ISR,
> > 	and restore fpsimd state after ISR ends.
> > 	For use in external kernel module,
> > 	it is declared as EXPORT_SYMBOL.
> 
> This patch adds no in-tree modular users of this, so per the usual
> conventions, NAK to EXPORT_SYMBOL().

Ack, this looks supicious.  Can you explain why your usecase _requires_
FPSIMD in hardirq context?

For now, these functions are strictly for EFI use only and should never
be used by modules.

Cheers
---Dave