* Re: [PATCH] KVM: ppc: Fix size of the PSPB register
@ 2015-09-01 22:45 ` Paul Mackerras
0 siblings, 0 replies; 21+ messages in thread
From: Paul Mackerras @ 2015-09-01 22:45 UTC (permalink / raw)
To: Benjamin Herrenschmidt
Cc: Thomas Huth, linuxppc-dev, Alexander Graf, kvm-ppc, kvm
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.
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Linuxppc-dev@lists.ozlabs.org
https://lists.ozlabs.org/listinfo/linuxppc-dev
^ permalink raw reply [flat|nested] 21+ messages in thread
* Re: [PATCH] KVM: ppc: Fix size of the PSPB register
@ 2015-09-01 22:45 ` Paul Mackerras
0 siblings, 0 replies; 21+ messages in thread
From: Paul Mackerras @ 2015-09-01 22:45 UTC (permalink / raw)
To: Benjamin Herrenschmidt
Cc: Thomas Huth, kvm-ppc, Alexander Graf, linuxppc-dev, kvm
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.
^ permalink raw reply [flat|nested] 21+ messages in thread
* Re: [PATCH] KVM: ppc: Fix size of the PSPB register
2015-09-01 22:45 ` Paul Mackerras
@ 2015-09-01 22:55 ` Benjamin Herrenschmidt
-1 siblings, 0 replies; 21+ messages in thread
From: Benjamin Herrenschmidt @ 2015-09-01 22:55 UTC (permalink / raw)
To: Paul Mackerras; +Cc: Thomas Huth, kvm-ppc, Alexander Graf, linuxppc-dev, kvm
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.
^ permalink raw reply [flat|nested] 21+ messages in thread
* Re: [PATCH] KVM: ppc: Fix size of the PSPB register
@ 2015-09-01 22:55 ` Benjamin Herrenschmidt
0 siblings, 0 replies; 21+ messages in thread
From: Benjamin Herrenschmidt @ 2015-09-01 22:55 UTC (permalink / raw)
To: Paul Mackerras; +Cc: Thomas Huth, kvm-ppc, Alexander Graf, linuxppc-dev, kvm
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.
^ permalink raw reply [flat|nested] 21+ messages in thread
* Re: [PATCH] KVM: ppc: Fix size of the PSPB register
2015-09-01 22:55 ` Benjamin Herrenschmidt
@ 2015-09-02 7:26 ` Thomas Huth
-1 siblings, 0 replies; 21+ messages in thread
From: Thomas Huth @ 2015-09-02 7:26 UTC (permalink / raw)
To: benh, Paul Mackerras; +Cc: kvm-ppc, Alexander Graf, linuxppc-dev, kvm
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
^ permalink raw reply [flat|nested] 21+ messages in thread
* Re: [PATCH] KVM: ppc: Fix size of the PSPB register
@ 2015-09-02 7:26 ` Thomas Huth
0 siblings, 0 replies; 21+ messages in thread
From: Thomas Huth @ 2015-09-02 7:26 UTC (permalink / raw)
To: benh, Paul Mackerras; +Cc: kvm-ppc, Alexander Graf, linuxppc-dev, kvm
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
^ permalink raw reply [flat|nested] 21+ messages in thread
* Re: [PATCH] KVM: ppc: Fix size of the PSPB register
2015-09-02 7:26 ` Thomas Huth
@ 2015-09-02 8:26 ` Alexander Graf
-1 siblings, 0 replies; 21+ messages in thread
From: Alexander Graf @ 2015-09-02 8:26 UTC (permalink / raw)
To: Thomas Huth
Cc: benh@au1.ibm.com, Paul Mackerras, kvm-ppc@vger.kernel.org,
Alexander Graf, linuxppc-dev@lists.ozlabs.org,
kvm@vger.kernel.org
> Am 02.09.2015 um 09:26 schrieb Thomas Huth <thuth@redhat.com>:
>
>> 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.
Sounds like a great candidate for kvm-unit-tests then, no? ;)
Alex
^ permalink raw reply [flat|nested] 21+ messages in thread
* Re: [PATCH] KVM: ppc: Fix size of the PSPB register
@ 2015-09-02 8:26 ` Alexander Graf
0 siblings, 0 replies; 21+ messages in thread
From: Alexander Graf @ 2015-09-02 8:26 UTC (permalink / raw)
To: Thomas Huth
Cc: benh@au1.ibm.com, Paul Mackerras, kvm-ppc@vger.kernel.org,
Alexander Graf, linuxppc-dev@lists.ozlabs.org,
kvm@vger.kernel.org
> Am 02.09.2015 um 09:26 schrieb Thomas Huth <thuth@redhat.com>:
>
>> 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.
Sounds like a great candidate for kvm-unit-tests then, no? ;)
Alex
^ permalink raw reply [flat|nested] 21+ messages in thread
* Re: [PATCH] KVM: ppc: Fix size of the PSPB register
2015-09-02 8:26 ` Alexander Graf
@ 2015-09-02 8:35 ` Thomas Huth
-1 siblings, 0 replies; 21+ messages in thread
From: Thomas Huth @ 2015-09-02 8:35 UTC (permalink / raw)
To: Alexander Graf
Cc: benh@au1.ibm.com, Paul Mackerras, kvm-ppc@vger.kernel.org,
Alexander Graf, linuxppc-dev@lists.ozlabs.org,
kvm@vger.kernel.org, Andrew Jones
On 02/09/15 10:26, Alexander Graf wrote:
>
>> Am 02.09.2015 um 09:26 schrieb Thomas Huth <thuth@redhat.com>:
>>
>>> 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.
>
> Sounds like a great candidate for kvm-unit-tests then, no? ;)
I'm certainly looking forward to seeing powerpc support in there :-)
Thomas
^ permalink raw reply [flat|nested] 21+ messages in thread
* Re: [PATCH] KVM: ppc: Fix size of the PSPB register
@ 2015-09-02 8:35 ` Thomas Huth
0 siblings, 0 replies; 21+ messages in thread
From: Thomas Huth @ 2015-09-02 8:35 UTC (permalink / raw)
To: Alexander Graf
Cc: benh@au1.ibm.com, Paul Mackerras, kvm-ppc@vger.kernel.org,
Alexander Graf, linuxppc-dev@lists.ozlabs.org,
kvm@vger.kernel.org, Andrew Jones
On 02/09/15 10:26, Alexander Graf wrote:
>
>> Am 02.09.2015 um 09:26 schrieb Thomas Huth <thuth@redhat.com>:
>>
>>> 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.
>
> Sounds like a great candidate for kvm-unit-tests then, no? ;)
I'm certainly looking forward to seeing powerpc support in there :-)
Thomas
^ permalink raw reply [flat|nested] 21+ messages in thread