linux-kernel - Re: [PATCH v2 2/3] MIPS: Setup an instruction emulation in VDSO protected page instead of user stack

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Date:	Fri, 10 Oct 2014 16:40:26 -0700
From:	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>
To:	David Daney <ddaney.cavm@...il.com>
CC:	James Hogan <james.hogan@...tec.com>, <linux-mips@...ux-mips.org>,
	<Zubair.Kakakhel@...tec.com>, <geert+renesas@...der.be>,
	<david.daney@...ium.com>, <peterz@...radead.org>,
	<paul.gortmaker@...driver.com>, <davidlohr@...com>,
	<macro@...ux-mips.org>, <chenhc@...ote.com>, <richard@....at>,
	<zajec5@...il.com>, <keescook@...omium.org>,
	<alex@...x-smith.me.uk>, <tglx@...utronix.de>,
	<blogic@...nwrt.org>, <jchandra@...adcom.com>,
	<paul.burton@...tec.com>, <qais.yousef@...tec.com>,
	<linux-kernel@...r.kernel.org>, <ralf@...ux-mips.org>,
	<markos.chandras@...tec.com>, <dengcheng.zhu@...tec.com>,
	<manuel.lauss@...il.com>, <akpm@...ux-foundation.org>,
	<lars.persson@...s.com>
Subject: Re: [PATCH v2 2/3] MIPS: Setup an instruction emulation in VDSO protected
 page instead of user stack

On 10/10/2014 03:56 PM, David Daney wrote:
>
>>  > Right, look at uprobes, it does exactly all this with a single page.
>>  > Slot allocation will block waiting for a free slot when all are in 
>> use.
>>
>> I don't see a reason to change my 300 lines design into much more
>> lengthy code. That code has more links to the rest of kernel and high
>> possibility to execute atomic operation/locks/mutex/etc - I can't do it
>> for emulation of MIPS locking instructions.
>>
>
> It isn't just the number of lines of code that is important.
>
> Doesn't your solution consume an extra page for each thread requiring 
> emulation?  That could be a significant amount of memory in a system 
> with many threads.

Yes, you right. However, per-thread memory is useful for many goals.

>
> Are you are using this to emulate atomic operations in addition to FPU 
> branch delay slot instructions?  Where is the code that does that?

Yes, in MIPS R2 emulator - MIPS R6 changed LL/LLD/SC/SCD opcodes and 
offset size.
The code-in-developement is in 
ssh://git.linux-mips.org/pub/scm/yegoshin/mips.git, branch 
android-linux-mti-3.10.14

- Leonid.
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