Re: [RFC PATCH v2 7/8] arm64/sve: Don't disable SVE on syscalls return

From: Dave Martin <Dave.Martin@arm.com>
To: Julien Grall <julien.grall@arm.com>
Cc: Anton.Kirilov@arm.com, catalin.marinas@arm.com,
	will.deacon@arm.com, oleg@redhat.com, zhang.lei@jp.fujitsu.com,
	alex.bennee@linaro.org, linux-arm-kernel@lists.infradead.org,
	Daniel.Kiss@arm.com
Subject: Re: [RFC PATCH v2 7/8] arm64/sve: Don't disable SVE on syscalls return
Date: Tue, 25 Jun 2019 10:41:51 +0100	[thread overview]
Message-ID: <20190625094150.GY2790@e103592.cambridge.arm.com> (raw)
In-Reply-To: <54e5e484-0268-0593-e5e7-16d7451cc849@arm.com>

On Mon, Jun 24, 2019 at 05:44:53PM +0100, Julien Grall wrote:
> Hi Dave,
> 
> I will answer the rest of the e-mail separately.
> 
> On 6/21/19 4:33 PM, Dave Martin wrote:
> >On Thu, Jun 13, 2019 at 05:16:55PM +0100, Julien Grall wrote:
> >>+	 * state on context switch. The bit will be set again while
> >>+	 * restoring/zeroing the registers.
> >>  	 */
> >>-	sve_user_disable();
> >>+	if (test_and_clear_thread_flag(TIF_SVE))
> >>+		set_thread_flag(TIF_SVE_NEEDS_FLUSH);
> >
> >Do we need to do this under local_bh_disable()?  Does it matter?
> >
> >It looks to me like this is called when we still have interrupts
> >disabled, which is probably worth fixing.  This may actually be a bug:
> >if kzalloc() causes us to sleep, we have the potential to trigger
> >sleeping-while-atomic splats.
> 
> I am not sure to understand this. Which kzalloc do you refer to? Is it the
> one in sve_alloc?
> 
> >
> >If you agree that's a bug, try to come up with a preliminary patch
> >that's suitable for stable.
> >
> >(I may have just misunderstood / misremembered something here.)
> 
> Cheers,

It looks like I completely confused myself here.

For some reason I though this was part of the do_sve_acc() path, which
is nonsense.  So please ignore that comment!

do_sve_acc() is indeed called with interrupts enabled.

Cheers
---Dave

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