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

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Date:   Mon, 8 Jun 2020 11:34:00 +0100
From:   Dave Martin <Dave.Martin@....com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Wooyeon Kim <wooy88.kim@...sung.com>, hk92.kim@...sung.com,
        Catalin Marinas <catalin.marinas@....com>,
        Bhupesh Sharma <bhsharma@...hat.com>, yb.song@...sung.com,
        yj.yim@...sung.com, Julien Grall <julien.grall@....com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Will Deacon <will@...nel.org>, jinsoo37.kim@...sung.com,
        hyewon.ryu@...sung.com, yhwan.joo@...sung.com,
        Anisse Astier <aastier@...ebox.fr>,
        Marc Zyngier <maz@...nel.org>,
        Allison Randal <allison@...utok.net>, dongww.kim@...sung.com,
        Sanghoon Lee <shoon114.lee@...sung.com>, jihun.kim@...sung.com,
        hyeyeon5.shim@...sung.com, Kees Cook <keescook@...omium.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Wooki Min <wooki.min@...sung.com>,
        Kristina Martsenko <kristina.martsenko@....com>,
        junik.lee@...sung.com, sgun.bae@...sung.com,
        Jeongtae Park <jtp.park@...sung.com>, kgene.kim@...sung.com,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-arm-kernel@...ts.infradead.org,
        Steve Capper <steve.capper@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, James Morse <james.morse@....com>,
        Sudeep Holla <sudeep.holla@....com>, dh.han@...sung.com
Subject: Re: [PATCH] arm64: fpsimd: Added API to manage fpsimd state inside
 kernel

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@...sung.com>
> > 
> >      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