linux-kernel - Re: [PATCH 4/4] hvc_dcc: disable user-space access to DCC

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Date:	Fri, 3 Jul 2015 10:53:12 -0400
From:	Timur Tabi <timur@...eaurora.org>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Shanker Donthineni <shankerd@...eaurora.org>,
	awallis@...eaurora.org, abhimany@...eaurora.org,
	will.deacon@....com, Vipul Gandhi <vgandhi@...eaurora.org>
Subject: Re: [PATCH 4/4] hvc_dcc: disable user-space access to DCC


On Jul 1, 2015, at 8:00 PM, Stephen Boyd wrote:

>> 
>> #include <asm/barrier.h>
>> +#include <asm/hardware/cp14.h>
> 
> That's new.

And no one else is using it either.

>> 
>> +
>> +static inline void __dcc_initialize(void)
>> +{
>> +	u32 val;
>> +
>> +	/* Disable user-space access to DCC */
>> +	val = MRC14(0, c0, c1, 0);
>> +	val |= 1 << 12; /* DSCR[Comms] */
>> +	MCR14(val, 0, c0, c1, 0);
>> +}
> 
> Nitpick: We do raw cp14 accesses for other functions in this file, so it
> would be nice to be consistent throughout.

I agree, but I didn't want to expand the scope of this patch beyond what it's supposed to do.

>> 
>> +static inline void __dcc_initialize(void)
>> +{
>> +	u32 val;
>> +
>> +	/* Disable user-space access to DCC */
>> +	asm ("mrs %0, mdscr_el1\n"
> 
> shouldn't this be volatile?

Well, I figured it wasn't important exactly why these instructions are executed, since it just disables user-space.  I usually get chewed out by someone if I add volatile where it's not needed, so I didn't add it this time.

> 
>> +		"	orr %0, %0, #4096\n" /* Set the TDCC bit */
> 
> arm64 has some #defines in debug-monitors.h which we should probably
> extend to have this bit as a #define too.

Hmmm, the only thing I see in debug_monitors.h that seems applicable are the mdscr_write() and mdscr_read() functions, which seem to overlap cp14.h.

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