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Date:   Wed, 20 Sep 2017 16:07:17 +0200
From:   Fredrik Noring <noring@nocrew.org>
To:     "Maciej W. Rozycki" <macro@imgtec.com>
Cc:     linux-mips@linux-mips.org
Subject: Re: [PATCH v2] MIPS: Add basic R5900 support
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Hi Maciej,

>  Can you add a diagnostic consistency check to the context restoration 
> code, i.e. all the macros called from RESTORE_ALL (in <asm/stackframe.h>), 
> such as a `break 12' (BRK_BUG) instruction if a register value is not 
> correctly sign-extended?

Hmm... I think some details need to be sorted out for this. The LW
instruction used to restore registers sign-extends to register length by
definition (p. A-70 in the TX79 manual), so I assume that isn't what we
are going to check unless we suspect a grave hardware error with LW? (Do
we need to check the register values immediately prior to LW?)

Another possibility would be to check that saved registers in SAVE_ALL
will be restored properly. That is, immediately after SW check that LW
(to a temporary register such as k1) will restore to the same value by
64-bit comparison and trap if unequal (TNE). I thought that made sense.
Something like for example

	sw	$17, PT_R17(sp)
	lw	k1, PT_R17(sp)
	tne	k1, $17, 12

as a replacement for

	LONG_S	$17, PT_R17(sp)

in SAVE_STATIC?

A question is whether registers are clobbered within the kernel itself
(via interrupts or some such) or for user programs.

Fredrik