linux-kernel - Re: [PATCH RFC v1] arm64: Handle traps from accessing CNTVCT/CNTFRQ for CONFIG

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Date:   Tue, 16 Jan 2018 12:32:19 -0800
From:   Nicolin Chen <nicoleotsuka@...il.com>
To:     Marc Zyngier <marc.zyngier@....com>
Cc:     mark.rutland@....com, catalin.marinas@....com, will.deacon@....com,
        oleg@...hat.com, cdall@...aro.org, tbaicar@...eaurora.org,
        julien.thierry@....com, Dave.Martin@....com, robin.murphy@....com,
        james.morse@....com, ard.biesheuvel@...aro.org,
        xiexiuqi@...wei.com, mingo@...nel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC v1] arm64: Handle traps from accessing CNTVCT/CNTFRQ
 for CONFIG_COMPAT

Hello Marc,

On Thu, Jan 11, 2018 at 08:51:37AM +0000, Marc Zyngier wrote:
> > [ I also added cntfrq here for safety as theoretically it could
> >   trigger the trap as well. However, my another test case (with
> >   mrc insturction) doesn't seem to trigger a trap. So I would
> >   drop it in the next version if someone can confirm it's not
> >   required. Thanks -- Nicolin ]
> 
> See my previous series on this very subject[1] as well as Will's reply.

Thanks for the background.

> > -	for (hook = sys64_hooks; hook->handler; hook++)
> > +	for (; hook && hook->handler; hook++)
> >  		if ((hook->esr_mask & esr) == hook->esr_val) {
> >  			hook->handler(esr, regs);
> >  			return;
> > 
> 
> Also, this code is fairly broken in its handling of conditional
> instructions.

I understand that it should take care of the condition field as
a general instruction handler. Just for curiosity: If we confine
the topic to read access of CNTVCT/CNTFRQ, what'd be the penalty
by ignoring the condition field and executing it anyway?

Thank you
Nicolin