linux-kernel - Re: [PATCH v4 12/19] ARM: LPAE: Add context switching support

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Date:	Sat, 19 Feb 2011 23:16:56 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 12/19] ARM: LPAE: Add context switching support

On Saturday, 19 February 2011, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:
> On Mon, Feb 14, 2011 at 01:24:06PM +0000, Catalin Marinas wrote:
>> On Sat, 2011-02-12 at 10:44 +0000, Russell King - ARM Linux wrote:
>> > On Mon, Jan 24, 2011 at 05:55:54PM +0000, Catalin Marinas wrote:
>> > > +#ifdef CONFIG_ARM_LPAE
>> > > +#define cpu_set_asid(asid) {                                         \
>> > > +     unsigned long ttbl, ttbh;                                       \
>> > > +     asm("   mrrc    p15, 0, %0, %1, c2              @ read TTBR0\n" \
>> > > +         "   mov     %1, %1, lsl #(48 - 32)          @ set ASID\n"   \
>> > > +         "   mcrr    p15, 0, %0, %1, c2              @ set TTBR0\n"  \
>> > > +         : "=r" (ttbl), "=r" (ttbh)                                  \
>> > > +         : "r" (asid & ~ASID_MASK));                                 \
>> >
>> > This is wrong:
>> > 1. It does nothing with %2 (the new asid)
>> > 2. it shifts the high address bits of TTBR0 left 16 places each time its
>> >    called.
>>
>> It was worse actually, not even compiled in because it had output
>> arguments but it wasn't volatile. Some early clobber is also needed.
>> What about this:
>>
>> #define cpu_set_asid(asid) {                                          \
>>       unsigned long ttbl, ttbh;                                       \
>>       asm volatile(                                                   \
>>       "       mrrc    p15, 0, %0, %1, c2              @ read TTBR0\n" \
>>       "       mov     %1, %2, lsl #(48 - 32)          @ set ASID\n"   \
>>       "       mcrr    p15, 0, %0, %1, c2              @ set TTBR0\n"  \
>>       : "=&r" (ttbl), "=&r" (ttbh)                                    \
>>       : "r" (asid & ~ASID_MASK));                                     \
>> }
>
> So we don't care about the low 16 bits of ttbh which can be simply zeroed?

Since the pgd is always allocated from lowmem, it is within 32-bit of
physical address and we can safely ignore ttbh. I could write a
comment here to this.

Catalin

-- 
Catalin