linux-kernel - Re: [PATCH 2/3] MIPS: Add full ISA emulator.

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Message-ID: <547FB8FB.7040803@caviumnetworks.com>
Date:	Wed, 3 Dec 2014 17:29:31 -0800
From:	David Daney <ddaney@...iumnetworks.com>
To:	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>
CC:	David Daney <ddaney.cavm@...il.com>, <linux-mips@...ux-mips.org>,
	<ralf@...ux-mips.org>, <Zubair.Kakakhel@...tec.com>,
	<geert+renesas@...der.be>, <peterz@...radead.org>,
	<paul.gortmaker@...driver.com>, <macro@...ux-mips.org>,
	<chenhc@...ote.com>, <cl@...ux.com>, <mingo@...nel.org>,
	<richard@....at>, <zajec5@...il.com>, <james.hogan@...tec.com>,
	<keescook@...omium.org>, <tj@...nel.org>, <alex@...x-smith.me.uk>,
	<pbonzini@...hat.com>, <blogic@...nwrt.org>,
	<paul.burton@...tec.com>, <qais.yousef@...tec.com>,
	<linux-kernel@...r.kernel.org>, <markos.chandras@...tec.com>,
	<dengcheng.zhu@...tec.com>, <manuel.lauss@...il.com>,
	<lars.persson@...s.com>, David Daney <david.daney@...ium.com>
Subject: Re: [PATCH 2/3] MIPS: Add full ISA emulator.

On 12/03/2014 04:52 PM, Leonid Yegoshin wrote:
> On 12/03/2014 04:20 PM, David Daney wrote:
>> On 12/03/2014 03:55 PM, Leonid Yegoshin wrote:
>>> On 12/03/2014 03:44 PM, David Daney wrote:
>>>
>>> (...)
>>>
>>> Big work
>>
>> Not really, although by number of lines of code, it is about 3x the
>> size of your patch, it only touches the existing code in one place. It
>> only took about 3 days to write, adding full MIPS64 and R6 support
>> would probably be less than another week of work.
>>
>> microMIPS I haven't looked at as we don't have anything to test it on.
>>
>>> but it doesn't support customized instructions,
>>
>> GCC will never put these in the delay slot of a FPU branch, so it is
>> not needed.
>
> I doubt that it is correct in all situations and with any GCC parameter
> combination.

My GCC experts assert that it is true.

>
> Never say never, if it is about toolchain. IMG Arch team was assured
> that branch likely are never used and removed it in MIPS R6, but BGEZL
> (or so) was a first which I hit then I ran GLIBC.

The fact that the Arch Team designed R6 without bothering to determine 
what legacy code does is not relevant to this conversation.

>
> Besides GCC there are LLVM and another JITs.
>

They don't do it either.  But that is not really important.  We can 
easily emulate faulting instructions if needed.

>>
>>> multiple ASEs,
>>
>> Same as above.  But any instructions that are deemed necessary can
>> easily be added.
>>
>>> MIPS R6
>>
>> It is a proof of concept.  R6 can easily be added if needed.
>>
>> Your XOL emulation doesn't handle R6 either, so this is no worse than
>> your patch in that respect.
>
> You probably didn't research it well. A lot of changes in
> arch/mips/kernel/branch.c and and arch/mips/math-emu/cp1emu.c, all of it
> related with R6.
>

I looked at:
commit 3a18ca061311f2f1ee9c44012f89c7436d392117

And I saw no R6 support.

Is it there, or in some other branch that isn't merged?


>>
>>> etc.
>>
>> GCC will never put trapping instructions in the delay slot either.
>
> It seems like it is not correct and requires a more accurate statement.
> FPU instructions may trap, LWL and LWR traps on R6 with RI, etc. Yes,
> there are restrictions but basing a kernel on that assumptions is
> unsafe. The only safe is HW architecture document.
>
> Finally, there is a manual encoding too.
>
>>
>> All we have to support are non-trapping and non-branch/jump
>> instructions from the ISA manuals that can be executed from userspace
>> processes. That makes it slightly simpler than complete ISA emulation.
>>
>>>
>>> Well, it is still not a replacement of XOL emulation.
>>
>> For use by the FPU emulator, it is probably good enough
>>
>>> Even close.
>>
>> I disagree, that is why I took the time to do it.
>>
>>>
>>>
>>
>

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