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:	Thu, 9 Oct 2014 16:10:15 -0700
From:	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>
To:	James Hogan <james.hogan@...tec.com>
CC:	<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

>> Small stack of emulation blocks is supported because nested traps are possible
>> in MIPS32/64 R6 emulation mix with FPU emulation.
> Could you please clarify how this nesting of emulation blocks could
> happen now that signals are handled more cleanly.
>
> I.e. isn't the emuframe stuff only required for instructions in branch
> delay slots, and branches shouldn't be in branch delay slots anyway, so
> I don't get how they could nest.
>
It may be a case for mix of FPU and MIPS R6 emulations. I just keep both 
emulators separate as much as possible but I assume that without prove 
it may be stackable - some rollback is needed to join both and it may 
(probably) cause a double emulation setup - dsemul may be called twice 
for the same pair of instructions. I didn't see that yet, honestly and 
you may be right.

And as for signals - it is a different issue, some signal may happen 
before or after emulated instruction in emulation block and I see that. 
But I see it only before because of probability for it is a lot of 
higher. Unwinding is need because signal handler may not return but 
longjump to somewhere.
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