Re: [Qemu-devel] [PATCH for-4.0 2/3] target/ppc: Enable "decrement and test CTR" version of bcctr

[David Gibson]

On Mon, Mar 25, 2019 at 12:25:15PM +0100, Greg Kurz wrote:
> On Mon, 25 Mar 2019 17:38:49 +1100
> David Gibson <address@hidden> wrote:
> 
> > On Fri, Mar 22, 2019 at 07:03:46PM +0100, Greg Kurz wrote:
> > > Even if all ISAs up to v3 indeed mention:
> > > 
> > >     If the "decrement and test CTR" option is specified (BO2=0), the
> > >     instruction form is invalid.
> > > 
> > > The UMs of all existing 64-bit server class processors say:  
> > 
> > I've applied this series because it fixes an immediate problem, but I
> > have some significant reservations about it, read on..
> > 
> > >     If BO[2] = 0, the contents of CTR (before any update) are used as the
> > >     target address and for the test of the contents of CTR to resolve the
> > >     branch. The contents of the CTR are then decremented and written back
> > >     to the CTR.  
> > 
> > So, if that's what the hardware does, I guess that's what we need to
> > do.  That behaviour seems totally bizarre though - how can it make
> > sense for the same register value to act as both the branch target and
> > a flag/counter?  Or am I misreading something?
> > 
> 
> I must confess this puzzles me as well :-\ ... and the linux commit that
> introduces the use of this opcode, ie. ee13cb249fab ("powerpc/64s: Add
> support for software count cache flush") doesn't provide any details,
> at least for someone ignorant like me :)

Oh... I suspect what's going on here is not actually that the branch
has that effect, but that they've re-used this nonsensical option
combination to mark the special cache count flush they need.  In which
case we'd need to not trap, but do something else in the TCG code.

> Maybe Michael can enlight us ?

Michael or Suraj..?

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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