Re: [PATCH] powerpc: book3s: kvm: Use the saved dsisr and dar values

[Paul Mackerras <paulus@samba.org>]

On Wed, Dec 18, 2013 at 10:44:08PM +0100, Alexander Graf wrote:
> 
> On 11.11.2013, at 15:02, Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com> wrote:
> 
> > From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
> > 
> > Don't try to compute these values.
> > 
> > Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
> > ---
> > 
> > NOTE: I am not sure why we were originally computing dsisr and dar. So may be
> > we need a variant of this patch. But with this and the additional patch
> > "powerpc: book3s: PR: Enable Little Endian PR guest" I am able to get a Little Endian
> > PR guest to boot.
> 
> It's quite easy to find out - git blame tells you all the history and points you to commit ca7f4203b.
> 
> commit ca7f4203b9b66e12d0d9968ff7dfe781f3a9695a
> Author: Alexander Graf <agraf@suse.de>
> Date:   Wed Mar 24 21:48:28 2010 +0100
> 
>     KVM: PPC: Implement alignment interrupt
> 
>     Mac OS X has some applications - namely the Finder - that require alignment
>     interrupts to work properly. So we need to implement them.
> 
>     But the spec for 970 and 750 also looks different. While 750 requires the
>     DSISR and DAR fields to reflect some instruction bits (DSISR) and the fault
>     address (DAR), the 970 declares this as an optional feature. So we need
>     to reconstruct DSISR and DAR manually.
> 
>     Signed-off-by: Alexander Graf <agraf@suse.de>
>     Signed-off-by: Avi Kivity <avi@redhat.com>
> 
> Read this as "on 970, alignment interrupts don't give us DSISR and DAR of the faulting instruction" as otherwise I wouldn't have implemented it.

Although it's optional, all IBM POWER cpus, and as far as I know all
PowerPC cpus, set DAR on an alignment interrupt to the effective
address being accessed.  You have a valid point regarding DSISR, but
it would be nice to skip the computations where either the host CPU
provides the bits, or the virtual CPU doesn't.

Paul.