[PATCH for-6.1 v4 0/1] machine: Disallow specifying topology parameters as zero

[PATCH for-6.1 v4 0/1] machine: Disallow specifying topology parameters as zero

[Yanan Wang]

In the SMP configuration, we should either specify a topology                       
parameter with a reasonable value (equal to or greater than 1)                      
or just leave it omitted and QEMU will calculate its value.                         
Configurations which explicitly specify the topology parameters                     
as zero like "sockets=0" are meaningless, so disallow them.                         
                                                                                    
However; the commit 1e63fe685804d                                                   
(machine: pass QAPI struct to mc->smp_parse) has documented that                    
'0' has the same semantics as omitting a parameter in the qapi                      
comment for SMPConfiguration. So this patch fixes the doc and                       
also adds the corresponding sanity check in the smp parsers.                        
                                                                                    
This patch originly comes form [1], and it was suggested that                       
this patch fixing the doc should be sent for 6.1 to avoid a                         
deprecation process in the future.                                                  
                                                                                    
[1] https://lore.kernel.org/qemu-devel/YPWsThPiZa3mF+zp@redhat.com/                 

v3->v4:
- fix the typos
- v3: https://lore.kernel.org/qemu-devel/20210723020002.18456-1-wangyanan55@huawei.com/
                                                                                    
v2->v3:                                                                             
- improve the error message                                                         
- v2: https://lore.kernel.org/qemu-devel/20210722154326.1464-1-wangyanan55@huawei.com/
                                                                                    
v1->v2:                                                                             
- move the check to machine_set_smp                                                 
- update qemu-option.hx                                                             
- v1: https://lore.kernel.org/qemu-devel/20210722021512.2600-1-wangyanan55@huawei.com/

Yanan Wang (1):
  machine: Disallow specifying topology parameters as zero

 hw/core/machine.c | 15 +++++++++++++++
 qapi/machine.json |  6 +++---
 qemu-options.hx   | 12 +++++++-----
 3 files changed, 25 insertions(+), 8 deletions(-)

-- 
2.19.1

[PATCH for-6.1 v4 1/1] machine: Disallow specifying topology parameters as zero

[Yanan Wang]

In the SMP configuration, we should either specify a topology
parameter with a reasonable value (equal to or greater than 1)
or just leave it omitted and QEMU will calculate its value.
Configurations which explicitly specify the topology parameters
as zero like "sockets=0" are meaningless, so disallow them.

However, the commit 1e63fe685804d
(machine: pass QAPI struct to mc->smp_parse) has documented that
'0' has the same semantics as omitting a parameter in the qapi
comment for SMPConfiguration. So this patch fixes the doc and
also adds the corresponding sanity check.

Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Tested-by: Daniel P. Berrange <berrange@redhat.com>
Suggested-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
---
 hw/core/machine.c | 15 +++++++++++++++
 qapi/machine.json |  6 +++---
 qemu-options.hx   | 12 +++++++-----
 3 files changed, 25 insertions(+), 8 deletions(-)

diff --git a/hw/core/machine.c b/hw/core/machine.c
index 775add0795..67f0429122 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -829,6 +829,21 @@ static void machine_set_smp(Object *obj, Visitor *v, const char *name,
         return;
     }
 
+    /*
+     * The topology parameters must be specified equal to or greater than one
+     * or just omitted, explicit configuration like "cpus=0" is not allowed.
+     */
+    if ((config->has_cpus && config->cpus == 0) ||
+        (config->has_sockets && config->sockets == 0) ||
+        (config->has_dies && config->dies == 0) ||
+        (config->has_cores && config->cores == 0) ||
+        (config->has_threads && config->threads == 0) ||
+        (config->has_maxcpus && config->maxcpus == 0)) {
+        error_setg(errp, "CPU topology parameters must be equal to "
+                   "or greater than one if provided");
+        goto out_free;
+    }
+
     mc->smp_parse(ms, config, errp);
     if (errp) {
         goto out_free;
diff --git a/qapi/machine.json b/qapi/machine.json
index c3210ee1fb..9272cb3cf8 100644
--- a/qapi/machine.json
+++ b/qapi/machine.json
@@ -1288,8 +1288,8 @@
 ##
 # @SMPConfiguration:
 #
-# Schema for CPU topology configuration.  "0" or a missing value lets
-# QEMU figure out a suitable value based on the ones that are provided.
+# Schema for CPU topology configuration. A missing value lets QEMU
+# figure out a suitable value based on the ones that are provided.
 #
 # @cpus: number of virtual CPUs in the virtual machine
 #
@@ -1297,7 +1297,7 @@
 #
 # @dies: number of dies per socket in the CPU topology
 #
-# @cores: number of cores per thread in the CPU topology
+# @cores: number of cores per die in the CPU topology
 #
 # @threads: number of threads per core in the CPU topology
 #
diff --git a/qemu-options.hx b/qemu-options.hx
index 99ed5ec5f1..323579da43 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -223,11 +223,13 @@ SRST
     of computing the CPU maximum count.
 
     Either the initial CPU count, or at least one of the topology parameters
-    must be specified. Values for any omitted parameters will be computed
-    from those which are given. Historically preference was given to the
-    coarsest topology parameters when computing missing values (ie sockets
-    preferred over cores, which were preferred over threads), however, this
-    behaviour is considered liable to change.
+    must be specified. The specified parameters must be equal to or greater
+    than one, explicit configuration like "cpus=0" is not allowed. Values
+    for any omitted parameters will be computed from those which are given.
+    Historically preference was given to the coarsest topology parameters
+    when computing missing values (ie sockets preferred over cores, which
+    were preferred over threads), however, this behaviour is considered
+    liable to change.
 ERST
 
 DEF("numa", HAS_ARG, QEMU_OPTION_numa,
-- 
2.19.1

Re: [PATCH for-6.1 v4 0/1] machine: Disallow specifying topology parameters as zero

[wangyanan (Y)]

Note: the latest v5 has been sent with minor text change. Hopefully that 
would be the last version.

Thanks,
Yanan

On 2021/7/23 15:31, Yanan Wang wrote:
> In the SMP configuration, we should either specify a topology
> parameter with a reasonable value (equal to or greater than 1)
> or just leave it omitted and QEMU will calculate its value.
> Configurations which explicitly specify the topology parameters
> as zero like "sockets=0" are meaningless, so disallow them.
>                                                                                      
> However; the commit 1e63fe685804d
> (machine: pass QAPI struct to mc->smp_parse) has documented that
> '0' has the same semantics as omitting a parameter in the qapi
> comment for SMPConfiguration. So this patch fixes the doc and
> also adds the corresponding sanity check in the smp parsers.
>                                                                                      
> This patch originly comes form [1], and it was suggested that
> this patch fixing the doc should be sent for 6.1 to avoid a
> deprecation process in the future.
>                                                                                      
> [1] https://lore.kernel.org/qemu-devel/YPWsThPiZa3mF+zp@redhat.com/
>
> v3->v4:
> - fix the typos
> - v3: https://lore.kernel.org/qemu-devel/20210723020002.18456-1-wangyanan55@huawei.com/
>                                                                                      
> v2->v3:
> - improve the error message
> - v2: https://lore.kernel.org/qemu-devel/20210722154326.1464-1-wangyanan55@huawei.com/
>                                                                                      
> v1->v2:
> - move the check to machine_set_smp
> - update qemu-option.hx
> - v1: https://lore.kernel.org/qemu-devel/20210722021512.2600-1-wangyanan55@huawei.com/
>
> Yanan Wang (1):
>    machine: Disallow specifying topology parameters as zero
>
>   hw/core/machine.c | 15 +++++++++++++++
>   qapi/machine.json |  6 +++---
>   qemu-options.hx   | 12 +++++++-----
>   3 files changed, 25 insertions(+), 8 deletions(-)
>