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. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
