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Date:   Fri, 17 Nov 2017 08:35:56 +0100
From:   Christoffer Dall <cdall@linaro.org>
To:     "Liuwenliang (Abbott Liu)" <liuwenliang@huawei.com>
Cc:     Marc Zyngier <marc.zyngier@arm.com>,
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Subject: Re: [PATCH 01/11] Initialize the mapping of KASan shadow memory
Message-ID: <20171117073556.GB28855@cbox>
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On Fri, Nov 17, 2017 at 07:18:45AM +0000, Liuwenliang (Abbott Liu) wrote:
> On 16/11/17  22:41 Marc Zyngier [mailto:marc.zyngier@arm.com] wrote:
> >No, it doesn't. It cannot work, because Cortex-A9 predates the invention
> >of the 64bit accessor. I suspect that you are testing stuff in QEMU,
> >which is giving you a SW model that always supports LPAE. I suggest you
> >test this code on *real* HW, and not only on QEMU.
> 
> I am sorry. My test is fault. I only defined TTBR0 as __ACCESS_CP15_64,
> but I don't use the definition TTBR0 as __ACCESS_CP15_64. 
> 
> Now I use the definition TTBR0 as __ACCESS_CP15_64 on CPU supporting
> LPAE(vexpress_a9)

What does a "CPU supporting LPAE(vexpress_a9) mean?  As Marc pointed
out, a Cortex-A9 doesn't support LPAE.  If you configure your kernel
with LPAE it's not going to work on a Cortex-A9.

> I find it doesn't work and report undefined instruction error
> when execute "mrrc" instruction.
> 
> So, you are right that 64bit accessor of TTBR0 cannot work on LPAE.
> 

It's the other way around.  It doesn't work WITHOUT LPAE, it only works
WITH LPAE.

The ARM ARM explains this quite clearly:

"Accessing TTBR0

To access TTBR0 in an implementation that does not include the Large
Physical Address Extension, or bits[31:0] of TTBR0 in an implementation
that includes the Large Physical Address Extension, software reads or
writes the CP15 registers with <opc1> set to 0, <CRn> set to c2, <CRm>
set to c0, and <opc2> set to 0. For example:

MRC p15, 0, <Rt>, c2, c0, 0
  ; Read 32-bit TTBR0 into Rt
MCR p15, 0, <Rt>, c2, c0, 0
  ; Write Rt to 32-bit TTBR0

In an implementation that includes the Large Physical Address Extension,
to access all 64 bits of TTBR0, software performs a 64-bit read or write
of the CP15 registers with <CRm> set to c2 and <opc1> set to 0. For
example:

MRRC p15, 0, <Rt>, <Rt2>, c2
  ; Read 64-bit TTBR0 into Rt (low word) and Rt2 (high word)
MCRR p15, 0, <Rt>, <Rt2>, c2
  ; Write Rt (low word) and Rt2 (high word) to 64-bit TTBR0

In these MRRC and MCRR instructions, Rt holds the least-significant word
of TTBR0, and Rt2 holds the most-significant word."

That is, if your processor (like the Cortex-A9) does NOT support LPAE,
all you have is the 32-bit accessors (MRC and MCR).

If your processor does support LPAE (like a Cortex-A15 for example),
then you have both the 32-bit accessors (MRC and MCR) and the 64-bit
accessors (MRRC, MCRR), and using the 32-bit accessor will simply access
the lower 32-bits of the 64-bit register.

Hope this helps,
-Christoffer

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