linux-kernel - Re: [PATCH v2 2/2] arm64: enable context tracking

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Message-ID: <20140508102514.GA6942@arm.com>
Date:	Thu, 8 May 2014 11:25:14 +0100
From:	Will Deacon <will.deacon@....com>
To:	Kevin Hilman <khilman@...aro.org>
Cc:	Larry Bassel <larry.bassel@...aro.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>
Subject: Re: [PATCH v2 2/2] arm64: enable context tracking

On Thu, May 08, 2014 at 12:49:04AM +0100, Kevin Hilman wrote:
> Hi Will,

Hello Kevin,

> Will Deacon <will.deacon@....com> writes:
> > On Wed, May 07, 2014 at 12:32:29AM +0100, Larry Bassel wrote:
> >> Make calls to ct_user_enter when the kernel is exited
> >> and ct_user_exit when the kernel is entered (in el0_da,
> >> el0_ia, el0_svc, el0_irq).
> >
> > Why only these entry points? I can reschedule after any exception from EL0,
> > so I'd expect all exceptions from userspace to need annotating, no?
> 
> In my initial approach to this, you might recall (though it was over a
> year ago now) was to just instrument kernel_enter rather than sprinkle
> the instrumentaion in cl0_*.  However, your concern at the time was that
> since it was before debugging was enabled it would complicate debugging
> these paths.
> 
> Any chance you have any other suggestion on how we might do this in
> kernel_entry rather than sprinkling them all over cl0_*?  or is the
> sprinkling the only good way to handle this.

Unfortunately, different exceptions do subtly different things before
invoking the main handler. For example:

 - Stashing the far
 - Enabling IRQs
 - Enabling debug
 - All the stuff on the syscall path

so putting the logic in kernel_entry isn't really do-able, unfortunately.

Will
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