[PATCH v2] hw/i386: Expand the range of CPU topologies between smp and maxcpus

[PATCH v2] hw/i386: Expand the range of CPU topologies between smp and maxcpus

[caodongli]

Change the criteria for the initial CPU topology and maxcpus, user can
have more settings

Signed-off-by: Dongli Cao <caodongli@kingsoft.com>
---
hw/i386/pc.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index 8a84b25..ef2e819 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -751,7 +751,7 @@ void pc_smp_parse(MachineState *ms, QemuOpts *opts)
             exit(1);
         }

-        if (sockets * dies * cores * threads != ms->smp.max_cpus) {
+        if (sockets * dies * cores * threads > ms->smp.max_cpus) {
             error_report("Invalid CPU topology deprecated: "
                          "sockets (%u) * dies (%u) * cores (%u) * threads (%u) "
                          "!= maxcpus (%u)",
--
1.8.3.1









caodongli@kingsoft.com

Re: [PATCH v2] hw/i386: Expand the range of CPU topologies between smp and maxcpus

[Daniel P. Berrangé]

On Mon, Apr 26, 2021 at 10:08:52AM +0800, caodongli@kingsoft.com wrote:
> Change the criteria for the initial CPU topology and maxcpus, user can
> have more settings

Can you provide a better explanation of why this is needed. What
valid usage scenario is blocked by the current check ?

AFAICT, it partially reverts an intentional change done in several
years ago in :


  commit bc1fb850a31468ac4976f3895f01a6d981e06d0a
  Author: Igor Mammedov <imammedo@redhat.com>
  Date:   Thu Sep 13 13:06:01 2018 +0200

    vl.c deprecate incorrect CPUs topology
    
    -smp [cpus],sockets/cores/threads[,maxcpus] should describe topology
    so that total number of logical CPUs [sockets * cores * threads]
    would be equal to [maxcpus], however historically we didn't have
    such check in QEMU and it is possible to start VM with an invalid
    topology.
    Deprecate invalid options combination so we can make sure that
    the topology VM started with is always correct in the future.
    Users with an invalid sockets/cores/threads/maxcpus values should
    fix their CLI to make sure that
       [sockets * cores * threads] == [maxcpus]



> 
> Signed-off-by: Dongli Cao <caodongli@kingsoft.com>
> ---
> hw/i386/pc.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> index 8a84b25..ef2e819 100644
> --- a/hw/i386/pc.c
> +++ b/hw/i386/pc.c
> @@ -751,7 +751,7 @@ void pc_smp_parse(MachineState *ms, QemuOpts *opts)
>              exit(1);
>          }
> 
> -        if (sockets * dies * cores * threads != ms->smp.max_cpus) {
> +        if (sockets * dies * cores * threads > ms->smp.max_cpus) {
>              error_report("Invalid CPU topology deprecated: "
>                           "sockets (%u) * dies (%u) * cores (%u) * threads (%u) "
>                           "!= maxcpus (%u)",

This is 

> --
> 1.8.3.1
> 
> 
> 
> 
> 
> 
> 
> 
> 
> caodongli@kingsoft.com
> 
> 

Regards,
Daniel
-- 
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Re: [PATCH v2] hw/i386: Expand the range of CPU topologies between smp and maxcpus

[Like Xu]

On 2021/4/26 21:30, Daniel P. Berrangé wrote:
> On Mon, Apr 26, 2021 at 10:08:52AM +0800, caodongli@kingsoft.com wrote:
>> Change the criteria for the initial CPU topology and maxcpus, user can
>> have more settings
> 
> Can you provide a better explanation of why this is needed. What
> valid usage scenario is blocked by the current check ?
> 
> AFAICT, it partially reverts an intentional change done in several
> years ago in :
> 
> 
>    commit bc1fb850a31468ac4976f3895f01a6d981e06d0a
>    Author: Igor Mammedov <imammedo@redhat.com>
>    Date:   Thu Sep 13 13:06:01 2018 +0200
> 
>      vl.c deprecate incorrect CPUs topology
>      
>      -smp [cpus],sockets/cores/threads[,maxcpus] should describe topology
>      so that total number of logical CPUs [sockets * cores * threads]
>      would be equal to [maxcpus], however historically we didn't have
>      such check in QEMU and it is possible to start VM with an invalid
>      topology.
>      Deprecate invalid options combination so we can make sure that
>      the topology VM started with is always correct in the future.
>      Users with an invalid sockets/cores/threads/maxcpus values should
>      fix their CLI to make sure that
>         [sockets * cores * threads] == [maxcpus]
> 
> 

Another helpful commit would be:

commit c4332cd1dcf2964c23893ab4c0bf8d774e42a3cf
Author: Igor Mammedov <imammedo@redhat.com>
Date:   Fri Sep 11 09:32:02 2020 -0400

     smp: drop support for deprecated (invalid topologies)

     it's was deprecated since 3.1

     Support for invalid topologies is removed, the user must ensure
     that topologies described with -smp include all possible cpus,
     i.e. (sockets * cores * threads) == maxcpus or QEMU will
     exit with error.


So is the following statement correct:

When we explicitly set the topology, we must ensure that the combination 
(sockets/dies/cores/threads/maxcpus) is always valid. If we need hot plug 
testing, we can only use something like "-smp 1,maxcpus = 4" since 3.1.

?


> 
>>
>> Signed-off-by: Dongli Cao <caodongli@kingsoft.com>
>> ---
>> hw/i386/pc.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/hw/i386/pc.c b/hw/i386/pc.c
>> index 8a84b25..ef2e819 100644
>> --- a/hw/i386/pc.c
>> +++ b/hw/i386/pc.c
>> @@ -751,7 +751,7 @@ void pc_smp_parse(MachineState *ms, QemuOpts *opts)
>>               exit(1);
>>           }
>>
>> -        if (sockets * dies * cores * threads != ms->smp.max_cpus) {
>> +        if (sockets * dies * cores * threads > ms->smp.max_cpus) {
>>               error_report("Invalid CPU topology deprecated: "
>>                            "sockets (%u) * dies (%u) * cores (%u) * threads (%u) "
>>                            "!= maxcpus (%u)",
> 
> This is
> 
>> --
>> 1.8.3.1
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> caodongli@kingsoft.com
>>
>>
> 
> Regards,
> Daniel
> 

Re: [PATCH v2] hw/i386: Expand the range of CPU topologies between smp and maxcpus

[Daniel P. Berrangé]

On Tue, Apr 27, 2021 at 10:13:01AM +0800, Like Xu wrote:
> On 2021/4/26 21:30, Daniel P. Berrangé wrote:
> > On Mon, Apr 26, 2021 at 10:08:52AM +0800, caodongli@kingsoft.com wrote:
> > > Change the criteria for the initial CPU topology and maxcpus, user can
> > > have more settings
> > 
> > Can you provide a better explanation of why this is needed. What
> > valid usage scenario is blocked by the current check ?
> > 
> > AFAICT, it partially reverts an intentional change done in several
> > years ago in :
> > 
> > 
> >    commit bc1fb850a31468ac4976f3895f01a6d981e06d0a
> >    Author: Igor Mammedov <imammedo@redhat.com>
> >    Date:   Thu Sep 13 13:06:01 2018 +0200
> > 
> >      vl.c deprecate incorrect CPUs topology
> >      -smp [cpus],sockets/cores/threads[,maxcpus] should describe topology
> >      so that total number of logical CPUs [sockets * cores * threads]
> >      would be equal to [maxcpus], however historically we didn't have
> >      such check in QEMU and it is possible to start VM with an invalid
> >      topology.
> >      Deprecate invalid options combination so we can make sure that
> >      the topology VM started with is always correct in the future.
> >      Users with an invalid sockets/cores/threads/maxcpus values should
> >      fix their CLI to make sure that
> >         [sockets * cores * threads] == [maxcpus]
> > 
> > 
> 
> Another helpful commit would be:
> 
> commit c4332cd1dcf2964c23893ab4c0bf8d774e42a3cf
> Author: Igor Mammedov <imammedo@redhat.com>
> Date:   Fri Sep 11 09:32:02 2020 -0400
> 
>     smp: drop support for deprecated (invalid topologies)
> 
>     it's was deprecated since 3.1
> 
>     Support for invalid topologies is removed, the user must ensure
>     that topologies described with -smp include all possible cpus,
>     i.e. (sockets * cores * threads) == maxcpus or QEMU will
>     exit with error.
> 
> 
> So is the following statement correct:
> 
> When we explicitly set the topology, we must ensure that the combination
> (sockets/dies/cores/threads/maxcpus) is always valid. If we need hot plug
> testing, we can only use something like "-smp 1,maxcpus = 4" since 3.1.

Yes, you must set maxcpus if you want ability to hotplug further
CPUs later.


Regards,
Daniel
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