Subject: Re: [PATCH 2/2] KVM: PPC: hypervisor large decrementer support

From: Michael Ellerman <[email-masked]>
Date: Tue, 07 Jun 2016 23:04:13 +1000

On Fri, 2016-06-03 at 07:46 +1000, Benjamin Herrenschmidt wrote:
> On Wed, 2016-06-01 at 16:23 +1000, Michael Neuling wrote:
> > FWIW you can use:
> >         andis. reg,reg,(LPCR_LD)@ha
> 
> @h in that case. Probably the same result but technically @ha is for
> arithmetic operations.

In this case it doesn't matter, because LPCR_LD is a single bit constant.

But if bit 15 of your constant happens to be 1, using @ha will mean 1 is added
to the high 16 bits of the value.

eg. if you have:

	#define CONSTANT        0x00208000

        lis     r4,CONSTANT@ha

You get:

	10000118:	21 00 80 3c 	lis     r4,33
	                                           ^
	                                           = 0x21

Documented (sort of) here:

  http://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi-1.9.html#RELOC-TYPE
  
  #ha(value) denotes the high adjusted value: bits 16 through 31 of the indicated value, compensating for #lo() being treated as a signed number:
  
      #ha(x) = (((x >> 16) + ((x & 0x8000) ? 1 : 0)) & 0xffff)

cheers