Re: [Qemu-devel] [Qemu-ppc] [PATCH] target/ppc: Stop parsing pvr list in H_CAS when exact match found

[Suraj Jitindar Singh <sjitindarsingh@gmail.com>]

On Mon, 2017-02-13 at 15:10 +1100, David Gibson wrote:
> On Mon, Feb 13, 2017 at 02:30:27PM +1100, Suraj Jitindar Singh wrote:
> > 
> > The pvr-list passed in H_Client_Architecture_Support is used to
> > communicate the supported pvrs of the client program. When an
> > exact match is found you are allowed to stop parsing the list and
> > continue
> > the boot process.
> > 
> > Currently while explicit_match is set when we find an exact match,
> > we still
> > set a compat mode based on best_compat irrespective of whether an
> > exact
> > match was found or not. This is wrong since it means we can only
> > ever run
> > in an architected state, not a raw state since we always set a
> > compat mode.
> > We are basically ignoring the case were we find an exact match.
> > 
> > Fix the code to stop parsing the pvr list when an exact match is
> > found.
> > This means that best_compat will always be zero in the case of an
> > exact
> > match which means we will not set a compat mode an thus run in raw
> > mode,
> > which is the desired functionality when we have an exact match.
> > 
> > Fixes: 152ef803ceb1 ("pseries: Rewrite CAS PVR compatibility
> > logic")
> > 
> > Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
> Nack.  That change was deliberately intended to prefer compatibility
> modes, only ever using a "raw" mode if there's no matching
> compatibility mode.
> 
> Using compatibility modes as often as possible makes migration across
> different host types substantially easier to handle and more likely
> to
> succeed.
Ok cool, didn't realise that was an intended effect. Guess I'll just
add power9 to the compat table.
> 
> > 
> > 
> > ---
> >  hw/ppc/spapr_hcall.c | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/hw/ppc/spapr_hcall.c b/hw/ppc/spapr_hcall.c
> > index 590105a..215c385 100644
> > --- a/hw/ppc/spapr_hcall.c
> > +++ b/hw/ppc/spapr_hcall.c
> > @@ -1028,6 +1028,7 @@ static target_ulong
> > h_client_architecture_support(PowerPCCPU *cpu,
> >  
> >          if ((cpu->env.spr[SPR_PVR] & pvr_mask) == (pvr &
> > pvr_mask)) {
> >              explicit_match = true;
> > +            break;
> >          } else {
> >              if (ppc_check_compat(cpu, pvr, best_compat,
> > max_compat)) {
> >                  best_compat = pvr;