Re: [PATCH V3 2/2] powerpc/kexec: Reset HILE before entering target kernel

On Fri, Jul 17, 2015 at 11:53:38AM +1000, Benjamin Herrenschmidt wrote:

On Fri, 2015-07-10 at 15:19 +1000, Samuel Mendoza-Jonas wrote:

quoted

+#if defined(CONFIG_PPC_BOOK3S_64) && defined(CONFIG_PPC_POWERNV)
+       li      r3,(FW_FEATURE_OPAL >> 16)
+       rldicr  r3,r3,16,63
+       and.    r3,r3,r26
+       cmpwi   r3,0
+       beq     99f

If FW_FEATRURE_OPAL is 0x80000000 then the li will sign extend.

The rldicr has a mask of all F's so it will keep all the bits you
don't care about.

So together, you'll get compares happening on bits above the 16 you care
about that might change the result of your comparison incorrectly.

Since FW_FEATURE_* bits aren't ABI, they can change, so we don't want
to impose a constraint on them.

Thus I would recommend using an rdlicl r3,r3,16,48 (aka srdi r3,r3,48)
instead which is going to clear all bits above 0xffff.

Or the other way around: instead of loading and masking, load zero and
OR the bits in:

+       li      r3,0
+       ori     r3,(FW_FEATURE_OPAL >> 16)
+       rldicr  r3,r3,16,63
+       and.    r3,r3,r26
+       cmpwi   r3,0
+       beq     99f

which of course is better written as

+       li      r3,0
+       oris    r3,(FW_FEATURE_OPAL >> 16)
+       and.    r3,r3,r26
+       cmpwi   r3,0
+       beq     99f

which is

+       andis.  r3,r26,(FW_FEATURE_OPAL >> 16)
+       cmpwi   r3,0
+       beq     99f

which is

+       andis.  r3,r26,(FW_FEATURE_OPAL >> 16)
+       beq     99f

Now, that being said, FW_FEATURE_* can be 64-bit

All of the code above only works for single-bit constants inside
0xffff0000 though (or 0x7fff0000 for the original).

and this isn't perf
critical so why not just load the full 64-bit constant into r3 and
be done with it ? There's a macro to do that:

	LOAD_REG_IMMEDIATE(r3,FW_FEATURE_OPAL)

And as you say later, use C.  Yeah.


Segher