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

From: Dave Martin <Dave.Martin@arm.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: hk92.kim@samsung.com, Catalin Marinas <catalin.marinas@arm.com>,
	Bhupesh Sharma <bhsharma@redhat.com>,
	yb.song@samsung.com, yj.yim@samsung.com,
	Julien Grall <julien.grall@arm.com>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Will Deacon <will@kernel.org>,
	jinsoo37.kim@samsung.com, hyewon.ryu@samsung.com,
	yhwan.joo@samsung.com, Anisse Astier <aastier@freebox.fr>,
	Marc Zyngier <maz@kernel.org>,
	dongww.kim@samsung.com, Allison Randal <allison@lohutok.net>,
	jihun.kim@samsung.com, Kees Cook <keescook@chromium.org>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Wooki Min <wooki.min@samsung.com>,
	Wooyeon Kim <wooy88.kim@samsung.com>,
	junik.lee@samsung.com, sgun.bae@samsung.com,
	Jeongtae Park <jtp.park@samsung.com>,
	kgene.kim@samsung.com,
	Kristina Martsenko <kristina.martsenko@arm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Sanghoon Lee <shoon114.lee@samsung.com>,
	linux-arm-kernel@lists.infradead.org,
	Sudeep Holla <sudeep.holla@arm.com>,
	Steve Capper <steve.capper@arm.com>,
	hyeyeon5.shim@samsung.com, linux-kernel@vger.kernel.org,
	James Morse <james.morse@arm.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	dh.han@samsung.com
Subject: Re: [PATCH] arm64: fpsimd: Added API to manage fpsimd state inside kernel
Date: Mon, 8 Jun 2020 11:34:00 +0100	[thread overview]
Message-ID: <20200608103340.GA31466@arm.com> (raw)
In-Reply-To: <20200605103705.GD85498@C02TD0UTHF1T.local>

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@samsung.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

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