linux-kernel - Re: questions about arm trustzone

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Date:	Tue, 25 Jan 2011 13:25:02 +0000
From:	Dave Martin <dave.martin@...aro.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Santosh Shilimkar <santosh.shilimkar@...com>,
	sen wang <wangchendu@...il.com>,
	David Brown <davidb@...eaurora.org>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: questions about arm trustzone

On Tue, Jan 25, 2011 at 12:45 PM, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:
> On Tue, Jan 25, 2011 at 12:24:13PM +0000, Dave Martin wrote:
>> Avoiding this complexity is one of the motivations for using r7 for
>> the syscall number with CONFIG_EABI (instead of using the SVC comment
>> field).
>
> Your history is not entirely correct.
>
> I had the kernel side of Thumb userspace support in place long before EABI
> came along.  Thumb doesn't have a large enough comment field to store the
> Linux syscall number, so to get around that problem, I decided to use r7
> for the syscall number.  You'll find 2.4 kernels support Thumb instructions
> in userspace.
>
> As part of the EABI switch for ARM mode - which created an incompatible
> SWI interface anyway, we decided that we could reduce data cache pollution
> by eliminating the read of the SWI instruction, so we adopted the r7
> method for ARM EABI mode.
>

Fair enough -- I was glossing over things a bit and I'm not familiar
with all the history.  So, I guess there were plenty of other good
reasons.

Cheers
---Dave
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