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Subject: Re: [PATCH] KVM: ppc: Fix size of the PSPB register
From: Benjamin Herrenschmidt <benh@au1.ibm.com>
Reply-To: benh@au1.ibm.com
To: Thomas Huth <thuth@redhat.com>, kvm-ppc@vger.kernel.org, Alexander Graf
 <agraf@suse.com>, Paul Mackerras <paulus@samba.org>
Cc: linuxppc-dev@lists.ozlabs.org, kvm@vger.kernel.org
Date: Wed, 02 Sep 2015 08:25:05 +1000
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On Tue, 2015-09-01 at 23:41 +0200, Thomas Huth wrote:
> The size of the Problem State Priority Boost Register is only
> 32 bits, so let's change the type of the corresponding variable
> accordingly to avoid future trouble.

It's not future trouble, it's broken today for LE and this should fix
it BUT ....

The asm accesses it using lwz/stw and C accesses it as a ulong. On LE
that will mean that userspace will see the value << 32

Now "fixing" it might break migration if that field is already
stored/loaded in its "broken" form. We may have to keep the "broken"
behaviour and document that qemu sees a value shifted by 32.

Cheers,
Ben.

> Signed-off-by: Thomas Huth <thuth@redhat.com>
> ---
>  arch/powerpc/include/asm/kvm_host.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/include/asm/kvm_host.h
> b/arch/powerpc/include/asm/kvm_host.h
> index d91f65b..c825f3a 100644
> --- a/arch/powerpc/include/asm/kvm_host.h
> +++ b/arch/powerpc/include/asm/kvm_host.h
> @@ -473,7 +473,7 @@ struct kvm_vcpu_arch {
>  	ulong ciabr;
>  	ulong cfar;
>  	ulong ppr;
> -	ulong pspb;
> +	u32 pspb;
>  	ulong fscr;
>  	ulong shadow_fscr;
>  	ulong ebbhr;