On 02/09/15 00:55, Benjamin Herrenschmidt wrote: > 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. Right ... I just meant that nobody really experienced trouble with this today yet, but the bug is already present now already of course. >>> 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. ... and if I get the QEMU source code right, the register is currently not migrated at all - or at least I was not able to find the spot in the source code that migrates this register. >> I think Thomas's patch is fine, it just needs a stronger patch >> description saying that it fixes an actual bug. Ok, I'll resend with a better patch description. Thomas -- To unsubscribe from this list: send the line "unsubscribe kvm-ppc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
