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

[David Gibson <david@gibson.dropbear.id.au>]

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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.

> 
> ---
>  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;

-- 
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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