Re: [PATCH v3 11/20] arm64: assembler: add macros to conditionally yield the NEON under PREEMPT

From: Dave Martin <Dave.Martin@arm.com>
To: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	Peter Zijlstra <peterz@infradead.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Will Deacon <will.deacon@arm.com>,
	Russell King - ARM Linux <linux@armlinux.org.uk>,
	Steven Rostedt <rostedt@goodmis.org>,
	"linux-crypto@vger.kernel.org" <linux-crypto@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>,
	linux-rt-users@vger.kernel.org
Subject: Re: [PATCH v3 11/20] arm64: assembler: add macros to conditionally yield the NEON under PREEMPT
Date: Thu, 7 Dec 2017 16:15:02 +0000	[thread overview]
Message-ID: <20171207161502.GI22781@e103592.cambridge.arm.com> (raw)
In-Reply-To: <CAKv+Gu-Otcg4UxhJA44PbbYamBJd26Be+YsSUQUaB+qy9owzvQ@mail.gmail.com>

On Thu, Dec 07, 2017 at 03:47:43PM +0000, Ard Biesheuvel wrote:
> On 7 December 2017 at 14:50, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
> > On 7 December 2017 at 14:39, Dave Martin <Dave.Martin@arm.com> wrote:
> >> On Wed, Dec 06, 2017 at 07:43:37PM +0000, Ard Biesheuvel wrote:

[...]

> >>> +     .macro          if_will_cond_yield_neon
> >>> +#ifdef CONFIG_PREEMPT
> >>> +     get_thread_info x0
> >>> +     ldr             w1, [x0, #TSK_TI_PREEMPT]
> >>> +     ldr             x0, [x0, #TSK_TI_FLAGS]
> >>> +     cmp             w1, #1 // == PREEMPT_OFFSET
> >>
> >> Can we at least drop a BUILD_BUG_ON() somewhere to check this?
> >>
> >> Maybe in kernel_neon_begin() since this is intimately kernel-mode NEON
> >> related.
> >>
> >
> > Sure.
> >
> 
> I only just understood your asm-offsets remark earlier. I wasn't aware
> that it allows exposing random constants as well (although it is
> fairly obvious now that I do). So I will expose PREEMPT_OFFSET rather
> than open code it

[...]

OK, yes, this works for any C expression that is compiletime-constant
but requires evaluation that the assembler doesn't understand.

Cheers
---Dave