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

[Paul Mackerras <paulus@ozlabs.org>]

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.

> 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.

> 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.

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

Paul.