[PATCH] KVM: ppc: Fix size of the PSPB register

[Benjamin Herrenschmidt]

On Wed, 2015-09-02 at 08:45 +1000, Paul Mackerras wrote:
> On Wed, Sep 02, 2015 at 08:25:05AM +1000, Benjamin Herrenschmidt
> wrote:
> > 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 ....
> 
> No, it's broken today for BE hosts, which will always see 0 for the
> PSPB register value.  LE hosts are fine.

0 or PSPB << 32 ?

> > 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
> 
> No, that will happen on BE, and since KVM_REG_PPC_PSPB says it's a
> 32-bit register, we'll just pass 0 back to userspace when it reads
> it.

Ah ok, I missed that bit about KVM_REG_PPC_PSPB

> > 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.
> 
> It will be being set to 0 on BE hosts across migration today
> (fortunately 0 is a benign value for PSPB).  If we fix this on both
> the source and destination host, then the value will get migrated
> across correctly.

Ok, I missed the part where KVM_REG_PPC_PSPB passed it down as a 32
-bit. That means Thomas patch should work indeed.

> I think Thomas's patch is fine, it just needs a stronger patch
> description saying that it fixes an actual bug.

Right.

Cheers,
Ben.