[Qemu-devel] [PATCH v3] spapr: set vsmt to MAX(8, smp_threads)

[Qemu-devel] [PATCH v3] spapr: set vsmt to MAX(8, smp_threads)

[Laurent Vivier]

We ignore silently the value of smp_threads when we set
the default VSMT value, and if smp_threads is greater than VSMT
kernel is going into trouble later.

Fixes: 8904e5a750
("spapr: Adjust default VSMT value for better migration compatibility")

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---

Notes:
    v3: use MAX(8, smp_threads) and let KVM to return an error
        if nb_threads is too big
        update subject to reflect the change
    
    v2: display a specific error message when the default VSMT is used
        fix subject

 hw/ppc/spapr.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index 32a876be56..c8a1eefa17 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -2310,7 +2310,7 @@ static void spapr_set_vsmt_mode(sPAPRMachineState *spapr, Error **errp)
          * the value that we'd get with KVM on POWER8, the
          * overwhelmingly common case in production systems.
          */
-        spapr->vsmt = 8;
+        spapr->vsmt = MAX(8, smp_threads);
     }
 
     /* KVM: If necessary, set the SMT mode: */
-- 
2.14.3

Re: [Qemu-devel] [PATCH v3] spapr: set vsmt to MAX(8, smp_threads)

[Greg Kurz]

On Fri,  9 Feb 2018 09:18:58 +0100
Laurent Vivier <lvivier@redhat.com> wrote:

> We ignore silently the value of smp_threads when we set
> the default VSMT value, and if smp_threads is greater than VSMT
> kernel is going into trouble later.
> 

Hi Laurent,

I've looked a bit more and I'm not sure what kernel troubles you're referring to,
but several places in QEMU where we use kvm_ppc_smt() later on do assume that
smp_threads > kvm_ppc_smt(). Basically, everywhere we compute a vCPU id:

In spapr_init_cpus() when creating DRC connectors:

        int core_id = i * smp_threads;

        if (mc->has_hotpluggable_cpus) {
            spapr_dr_connector_new(OBJECT(spapr), TYPE_SPAPR_DRC_CPU,
                                   (core_id / smp_threads) * smt);
        }

or in spapr_cpu_core_realize() when creating vCPUs:

        cpu->vcpu_id = (cc->core_id * spapr->vsmt / smp_threads) + i;

It is visible by adding some printfs in the current code base. This is what
happens when passing -smp cores=2,threads=16 without your patch:

DRC connector to vcpu_id 0
CPU vcpu_id 0
CPU vcpu_id 1
CPU vcpu_id 2
CPU vcpu_id 3
CPU vcpu_id 4
CPU vcpu_id 5
CPU vcpu_id 6
CPU vcpu_id 7
CPU vcpu_id 8
CPU vcpu_id 9
CPU vcpu_id 10
CPU vcpu_id 11
CPU vcpu_id 12
CPU vcpu_id 13
CPU vcpu_id 14
CPU vcpu_id 15
DRC connector to vcpu_id 8
                        ^^^
                     should be 16
CPU vcpu_id 8
           ^^^
   should start numbering at 16
CPU vcpu_id 9
CPU vcpu_id 10
CPU vcpu_id 11
CPU vcpu_id 12
CPU vcpu_id 13
CPU vcpu_id 14
CPU vcpu_id 15
CPU vcpu_id 16
CPU vcpu_id 17
CPU vcpu_id 18
CPU vcpu_id 19
CPU vcpu_id 20
CPU vcpu_id 21
CPU vcpu_id 22
CPU vcpu_id 23
qemu-system-ppc64: kvm_init_vcpu failed: File exists
                                              ^^^^
                                 CPU 8 already created by the first core

I'm not feeling comfortable with the rest of the code silently depending on
the fact that spapr_set_vsmt_mode() terminates QEMU if it cannot enforce
smp_threads <= kvm_ppc_smt().

Anyway, with your patch, the same command line as above gives:

qemu-system-ppc64: Failed to set KVM's VSMT mode to 16 (errno -22)
On PPC, a VM with 16 threads/core on a host with 8 threads/core requires
 the use of VSMT mode 16.
This KVM seems to be too old to support VSMT.

This hammer is big enough to fix the vCPU ids miscalculations, so:

Reviewed-by: Greg Kurz <groug@kaod.org>

> Fixes: 8904e5a750
> ("spapr: Adjust default VSMT value for better migration compatibility")
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
> 
> Notes:
>     v3: use MAX(8, smp_threads) and let KVM to return an error
>         if nb_threads is too big
>         update subject to reflect the change
>     
>     v2: display a specific error message when the default VSMT is used
>         fix subject
> 
>  hw/ppc/spapr.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index 32a876be56..c8a1eefa17 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -2310,7 +2310,7 @@ static void spapr_set_vsmt_mode(sPAPRMachineState *spapr, Error **errp)
>           * the value that we'd get with KVM on POWER8, the
>           * overwhelmingly common case in production systems.
>           */
> -        spapr->vsmt = 8;
> +        spapr->vsmt = MAX(8, smp_threads);
>      }
>  
>      /* KVM: If necessary, set the SMT mode: */

Re: [Qemu-devel] [PATCH v3] spapr: set vsmt to MAX(8, smp_threads)

[David Gibson]

[-- Attachment #1: Type: text/plain, Size: 4101 bytes --]

On Fri, Feb 09, 2018 at 03:06:49PM +0100, Greg Kurz wrote:
> On Fri,  9 Feb 2018 09:18:58 +0100
> Laurent Vivier <lvivier@redhat.com> wrote:
> 
> > We ignore silently the value of smp_threads when we set
> > the default VSMT value, and if smp_threads is greater than VSMT
> > kernel is going into trouble later.
> > 
> 
> Hi Laurent,
> 
> I've looked a bit more and I'm not sure what kernel troubles you're referring to,
> but several places in QEMU where we use kvm_ppc_smt() later on do assume that
> smp_threads > kvm_ppc_smt(). Basically, everywhere we compute a vCPU id:
> 
> In spapr_init_cpus() when creating DRC connectors:
> 
>         int core_id = i * smp_threads;
> 
>         if (mc->has_hotpluggable_cpus) {
>             spapr_dr_connector_new(OBJECT(spapr), TYPE_SPAPR_DRC_CPU,
>                                    (core_id / smp_threads) * smt);
>         }
> 
> or in spapr_cpu_core_realize() when creating vCPUs:
> 
>         cpu->vcpu_id = (cc->core_id * spapr->vsmt / smp_threads) + i;
> 
> It is visible by adding some printfs in the current code base. This is what
> happens when passing -smp cores=2,threads=16 without your patch:
> 
> DRC connector to vcpu_id 0
> CPU vcpu_id 0
> CPU vcpu_id 1
> CPU vcpu_id 2
> CPU vcpu_id 3
> CPU vcpu_id 4
> CPU vcpu_id 5
> CPU vcpu_id 6
> CPU vcpu_id 7
> CPU vcpu_id 8
> CPU vcpu_id 9
> CPU vcpu_id 10
> CPU vcpu_id 11
> CPU vcpu_id 12
> CPU vcpu_id 13
> CPU vcpu_id 14
> CPU vcpu_id 15
> DRC connector to vcpu_id 8
>                         ^^^
>                      should be 16
> CPU vcpu_id 8
>            ^^^
>    should start numbering at 16
> CPU vcpu_id 9
> CPU vcpu_id 10
> CPU vcpu_id 11
> CPU vcpu_id 12
> CPU vcpu_id 13
> CPU vcpu_id 14
> CPU vcpu_id 15
> CPU vcpu_id 16
> CPU vcpu_id 17
> CPU vcpu_id 18
> CPU vcpu_id 19
> CPU vcpu_id 20
> CPU vcpu_id 21
> CPU vcpu_id 22
> CPU vcpu_id 23
> qemu-system-ppc64: kvm_init_vcpu failed: File exists
>                                               ^^^^
>                                  CPU 8 already created by the first core
> 
> I'm not feeling comfortable with the rest of the code silently depending on
> the fact that spapr_set_vsmt_mode() terminates QEMU if it cannot enforce
> smp_threads <= kvm_ppc_smt().

I'm not quite sure what you're suggesting as an alternative, though.

> 
> Anyway, with your patch, the same command line as above gives:
> 
> qemu-system-ppc64: Failed to set KVM's VSMT mode to 16 (errno -22)
> On PPC, a VM with 16 threads/core on a host with 8 threads/core requires
>  the use of VSMT mode 16.
> This KVM seems to be too old to support VSMT.
> 
> This hammer is big enough to fix the vCPU ids miscalculations, so:
> 
> Reviewed-by: Greg Kurz <groug@kaod.org>
> 
> > Fixes: 8904e5a750
> > ("spapr: Adjust default VSMT value for better migration compatibility")
> > 
> > Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> > ---
> > 
> > Notes:
> >     v3: use MAX(8, smp_threads) and let KVM to return an error
> >         if nb_threads is too big
> >         update subject to reflect the change
> >     
> >     v2: display a specific error message when the default VSMT is used
> >         fix subject
> > 
> >  hw/ppc/spapr.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> > index 32a876be56..c8a1eefa17 100644
> > --- a/hw/ppc/spapr.c
> > +++ b/hw/ppc/spapr.c
> > @@ -2310,7 +2310,7 @@ static void spapr_set_vsmt_mode(sPAPRMachineState *spapr, Error **errp)
> >           * the value that we'd get with KVM on POWER8, the
> >           * overwhelmingly common case in production systems.
> >           */
> > -        spapr->vsmt = 8;
> > +        spapr->vsmt = MAX(8, smp_threads);
> >      }
> >  
> >      /* KVM: If necessary, set the SMT mode: */
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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Re: [Qemu-devel] [PATCH v3] spapr: set vsmt to MAX(8, smp_threads)

[Greg Kurz]

On Sat, 10 Feb 2018 20:23:07 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> On Fri, Feb 09, 2018 at 03:06:49PM +0100, Greg Kurz wrote:
> > On Fri,  9 Feb 2018 09:18:58 +0100
> > Laurent Vivier <lvivier@redhat.com> wrote:
> >   
> > > We ignore silently the value of smp_threads when we set
> > > the default VSMT value, and if smp_threads is greater than VSMT
> > > kernel is going into trouble later.
> > >   
> > 
> > Hi Laurent,
> > 
> > I've looked a bit more and I'm not sure what kernel troubles you're referring to,
> > but several places in QEMU where we use kvm_ppc_smt() later on do assume that
> > smp_threads > kvm_ppc_smt(). Basically, everywhere we compute a vCPU id:
> > 
> > In spapr_init_cpus() when creating DRC connectors:
> > 
> >         int core_id = i * smp_threads;
> > 
> >         if (mc->has_hotpluggable_cpus) {
> >             spapr_dr_connector_new(OBJECT(spapr), TYPE_SPAPR_DRC_CPU,
> >                                    (core_id / smp_threads) * smt);
> >         }
> > 
> > or in spapr_cpu_core_realize() when creating vCPUs:
> > 
> >         cpu->vcpu_id = (cc->core_id * spapr->vsmt / smp_threads) + i;
> > 
> > It is visible by adding some printfs in the current code base. This is what
> > happens when passing -smp cores=2,threads=16 without your patch:
> > 
> > DRC connector to vcpu_id 0
> > CPU vcpu_id 0
> > CPU vcpu_id 1
> > CPU vcpu_id 2
> > CPU vcpu_id 3
> > CPU vcpu_id 4
> > CPU vcpu_id 5
> > CPU vcpu_id 6
> > CPU vcpu_id 7
> > CPU vcpu_id 8
> > CPU vcpu_id 9
> > CPU vcpu_id 10
> > CPU vcpu_id 11
> > CPU vcpu_id 12
> > CPU vcpu_id 13
> > CPU vcpu_id 14
> > CPU vcpu_id 15
> > DRC connector to vcpu_id 8
> >                         ^^^
> >                      should be 16
> > CPU vcpu_id 8
> >            ^^^
> >    should start numbering at 16
> > CPU vcpu_id 9
> > CPU vcpu_id 10
> > CPU vcpu_id 11
> > CPU vcpu_id 12
> > CPU vcpu_id 13
> > CPU vcpu_id 14
> > CPU vcpu_id 15
> > CPU vcpu_id 16
> > CPU vcpu_id 17
> > CPU vcpu_id 18
> > CPU vcpu_id 19
> > CPU vcpu_id 20
> > CPU vcpu_id 21
> > CPU vcpu_id 22
> > CPU vcpu_id 23
> > qemu-system-ppc64: kvm_init_vcpu failed: File exists
> >                                               ^^^^
> >                                  CPU 8 already created by the first core
> > 
> > I'm not feeling comfortable with the rest of the code silently depending on
> > the fact that spapr_set_vsmt_mode() terminates QEMU if it cannot enforce
> > smp_threads <= kvm_ppc_smt().  
> 
> I'm not quite sure what you're suggesting as an alternative, though.
> 

I haven't suggested anything yet :)

But I was thinking of:
- having a single function to compute the vcpu_id, instead of open-coding
  the formula in several places like the current code does,
- this function should ensure all pre-requisites to compute the vcpu_id are
  met (including trying to set VSMT) or return an error

> > 
> > Anyway, with your patch, the same command line as above gives:
> > 
> > qemu-system-ppc64: Failed to set KVM's VSMT mode to 16 (errno -22)
> > On PPC, a VM with 16 threads/core on a host with 8 threads/core requires
> >  the use of VSMT mode 16.
> > This KVM seems to be too old to support VSMT.
> > 
> > This hammer is big enough to fix the vCPU ids miscalculations, so:
> > 
> > Reviewed-by: Greg Kurz <groug@kaod.org>
> >   
> > > Fixes: 8904e5a750
> > > ("spapr: Adjust default VSMT value for better migration compatibility")
> > > 
> > > Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> > > ---
> > > 
> > > Notes:
> > >     v3: use MAX(8, smp_threads) and let KVM to return an error
> > >         if nb_threads is too big
> > >         update subject to reflect the change
> > >     
> > >     v2: display a specific error message when the default VSMT is used
> > >         fix subject
> > > 
> > >  hw/ppc/spapr.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> > > index 32a876be56..c8a1eefa17 100644
> > > --- a/hw/ppc/spapr.c
> > > +++ b/hw/ppc/spapr.c
> > > @@ -2310,7 +2310,7 @@ static void spapr_set_vsmt_mode(sPAPRMachineState *spapr, Error **errp)
> > >           * the value that we'd get with KVM on POWER8, the
> > >           * overwhelmingly common case in production systems.
> > >           */
> > > -        spapr->vsmt = 8;
> > > +        spapr->vsmt = MAX(8, smp_threads);
> > >      }
> > >  
> > >      /* KVM: If necessary, set the SMT mode: */  
> >   
> 

Re: [Qemu-devel] [PATCH v3] spapr: set vsmt to MAX(8, smp_threads)

[David Gibson]

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On Mon, Feb 12, 2018 at 12:11:27PM +0100, Greg Kurz wrote:
> On Sat, 10 Feb 2018 20:23:07 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > On Fri, Feb 09, 2018 at 03:06:49PM +0100, Greg Kurz wrote:
> > > On Fri,  9 Feb 2018 09:18:58 +0100
> > > Laurent Vivier <lvivier@redhat.com> wrote:
> > >   
> > > > We ignore silently the value of smp_threads when we set
> > > > the default VSMT value, and if smp_threads is greater than VSMT
> > > > kernel is going into trouble later.
> > > >   
> > > 
> > > Hi Laurent,
> > > 
> > > I've looked a bit more and I'm not sure what kernel troubles you're referring to,
> > > but several places in QEMU where we use kvm_ppc_smt() later on do assume that
> > > smp_threads > kvm_ppc_smt(). Basically, everywhere we compute a vCPU id:
> > > 
> > > In spapr_init_cpus() when creating DRC connectors:
> > > 
> > >         int core_id = i * smp_threads;
> > > 
> > >         if (mc->has_hotpluggable_cpus) {
> > >             spapr_dr_connector_new(OBJECT(spapr), TYPE_SPAPR_DRC_CPU,
> > >                                    (core_id / smp_threads) * smt);
> > >         }
> > > 
> > > or in spapr_cpu_core_realize() when creating vCPUs:
> > > 
> > >         cpu->vcpu_id = (cc->core_id * spapr->vsmt / smp_threads) + i;
> > > 
> > > It is visible by adding some printfs in the current code base. This is what
> > > happens when passing -smp cores=2,threads=16 without your patch:
> > > 
> > > DRC connector to vcpu_id 0
> > > CPU vcpu_id 0
> > > CPU vcpu_id 1
> > > CPU vcpu_id 2
> > > CPU vcpu_id 3
> > > CPU vcpu_id 4
> > > CPU vcpu_id 5
> > > CPU vcpu_id 6
> > > CPU vcpu_id 7
> > > CPU vcpu_id 8
> > > CPU vcpu_id 9
> > > CPU vcpu_id 10
> > > CPU vcpu_id 11
> > > CPU vcpu_id 12
> > > CPU vcpu_id 13
> > > CPU vcpu_id 14
> > > CPU vcpu_id 15
> > > DRC connector to vcpu_id 8
> > >                         ^^^
> > >                      should be 16
> > > CPU vcpu_id 8
> > >            ^^^
> > >    should start numbering at 16
> > > CPU vcpu_id 9
> > > CPU vcpu_id 10
> > > CPU vcpu_id 11
> > > CPU vcpu_id 12
> > > CPU vcpu_id 13
> > > CPU vcpu_id 14
> > > CPU vcpu_id 15
> > > CPU vcpu_id 16
> > > CPU vcpu_id 17
> > > CPU vcpu_id 18
> > > CPU vcpu_id 19
> > > CPU vcpu_id 20
> > > CPU vcpu_id 21
> > > CPU vcpu_id 22
> > > CPU vcpu_id 23
> > > qemu-system-ppc64: kvm_init_vcpu failed: File exists
> > >                                               ^^^^
> > >                                  CPU 8 already created by the first core
> > > 
> > > I'm not feeling comfortable with the rest of the code silently depending on
> > > the fact that spapr_set_vsmt_mode() terminates QEMU if it cannot enforce
> > > smp_threads <= kvm_ppc_smt().  
> > 
> > I'm not quite sure what you're suggesting as an alternative, though.
> > 
> 
> I haven't suggested anything yet :)
> 
> But I was thinking of:
> - having a single function to compute the vcpu_id, instead of open-coding
>   the formula in several places like the current code does,

That seems like a good idea.

> - this function should ensure all pre-requisites to compute the vcpu_id are
>   met (including trying to set VSMT) or return an error

I'm much more dubious about this.  Checks that can be performed
passively I'm ok with, but I think something that looks like a simple
calculation helper shouldn't be having side-effects like poking at KVM
state.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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Re: [Qemu-devel] [PATCH v3] spapr: set vsmt to MAX(8, smp_threads)

[David Gibson]

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On Fri, Feb 09, 2018 at 09:18:58AM +0100, Laurent Vivier wrote:
> We ignore silently the value of smp_threads when we set
> the default VSMT value, and if smp_threads is greater than VSMT
> kernel is going into trouble later.
> 
> Fixes: 8904e5a750
> ("spapr: Adjust default VSMT value for better migration compatibility")
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>

Applied, thanks.

> ---
> 
> Notes:
>     v3: use MAX(8, smp_threads) and let KVM to return an error
>         if nb_threads is too big
>         update subject to reflect the change
>     
>     v2: display a specific error message when the default VSMT is used
>         fix subject
> 
>  hw/ppc/spapr.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index 32a876be56..c8a1eefa17 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -2310,7 +2310,7 @@ static void spapr_set_vsmt_mode(sPAPRMachineState *spapr, Error **errp)
>           * the value that we'd get with KVM on POWER8, the
>           * overwhelmingly common case in production systems.
>           */
> -        spapr->vsmt = 8;
> +        spapr->vsmt = MAX(8, smp_threads);
>      }
>  
>      /* KVM: If necessary, set the SMT mode: */

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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