From mboxrd@z Thu Jan 1 00:00:00 1970 From: Benjamin Herrenschmidt Date: Tue, 01 Sep 2015 22:55:07 +0000 Subject: Re: [PATCH] KVM: ppc: Fix size of the PSPB register Message-Id: <1441148107.2668.57.camel@au1.ibm.com> List-Id: References: <1441143678-14295-1-git-send-email-thuth@redhat.com> <1441146305.2668.51.camel@au1.ibm.com> <20150901224521.GC23007@fergus.ozlabs.ibm.com> In-Reply-To: <20150901224521.GC23007@fergus.ozlabs.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Paul Mackerras Cc: Thomas Huth , kvm-ppc@vger.kernel.org, Alexander Graf , linuxppc-dev@lists.ozlabs.org, kvm@vger.kernel.org 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.