Re: [Qemu-devel] [PATCH v7 01/13] machine: Don't allow CPU toplogies with partially filled cores

[Igor Mammedov]

On Fri, 29 Jan 2016 13:36:05 -0200
Eduardo Habkost <address@hidden> wrote:

> On Fri, Jan 29, 2016 at 04:10:47PM +0100, Igor Mammedov wrote:
> > On Fri, 29 Jan 2016 12:24:18 -0200
> > Eduardo Habkost <address@hidden> wrote:
> >   
> > > On Fri, Jan 29, 2016 at 02:52:30PM +1100, David Gibson wrote:  
> > > > On Thu, Jan 28, 2016 at 11:19:43AM +0530, Bharata B Rao wrote:    
> > > > > Prevent guests from booting with CPU topologies that have partially
> > > > > filled CPU cores or can result in partially filled CPU cores after
> > > > > CPU hotplug like
> > > > > 
> > > > > -smp 15,sockets=1,cores=4,threads=4,maxcpus=16 or
> > > > > -smp 15,sockets=1,cores=4,threads=4,maxcpus=17.
> > > > > 
> > > > > This is enforced by introducing MachineClass::validate_smp_config()
> > > > > that gets called from generic SMP parsing code. Machine type versions
> > > > > that want to enforce this can define this to the generic version
> > > > > provided.
> > > > > 
> > > > > Only sPAPR and PC machine types starting from version 2.6 enforce 
> > > > > this in
> > > > > this patch.
> > > > > 
> > > > > Signed-off-by: Bharata B Rao <address@hidden>    
> > > > 
> > > > I've been kind of lost in the back and forth about
> > > > threads/cores/sockets.
> > > > 
> > > > What, in the end, is the rationale for allowing partially filled
> > > > sockets, but not partially filled cores?    
> > > 
> > > I don't think there's a good reason for that (at least for PC).
> > > 
> > > It's easier to relax the requirements later if necessary, than
> > > dealing with compatibility issues again when making the code more
> > > strict. So I suggest we make validate_smp_config_generic() also
> > > check if smp_cpus % (smp_threads * smp_cores) == 0.  
> > 
> > that would break exiting setups.  
> 
> Not if we do that only on newer machine classes.
> validate_smp_config_generic() will be used only on *-2.6 and
> newer.
> 
> 
> > 
> > Also in case of cpu hotplug this patch will break migration
> > as target QEMU might refuse starting with hotplugged CPU thread.  
> 
> This won't change older machine-types.
> 
> But I think you are right: it can break migration on pc-2.6, too.
> But: isn't migration already broken when creating other sets of
> CPUs that can't represented using -smp?
> 
> How exactly would you migrate a machine today, if you run:
> 
>   $ qemu-system-x86_64 -smp 16,sockets=2,cores=2,threads=2,maxcpus=32
>   (QMP) cpu-add id=31
that's invalid topology and should exit with error at start-up,
however it shouldn't be smp_cpus vs sockets,cores,threads check
but rather max_cpus vs sockets,cores,threads,maxcpus check.
something like this:

diff --git a/vl.c b/vl.c
index f043009..3afa0b6 100644
--- a/vl.c
+++ b/vl.c
@@ -1239,9 +1239,9 @@ static void smp_parse(QemuOpts *opts)
         }
 
         max_cpus = qemu_opt_get_number(opts, "maxcpus", cpus);
-        if (sockets * cores * threads > max_cpus) {
-            error_report("cpu topology: "
-                         "sockets (%u) * cores (%u) * threads (%u) > "
+        if (sockets * cores * threads == max_cpus) {
+            error_report("invalid cpu topology: "
+                         "sockets (%u) * cores (%u) * threads (%u) not equal "
                          "maxcpus (%u)",
                          sockets, cores, threads, max_cpus);
             exit(1);

> 
> 
> > 
> > Perhaps this check should be enforced per target/machine if
> > arch requires it.  
> 
> It is. Please see the patch. It introduces a validate_smp_config
> method.
> 
> But we need your input to clarify if
> validate_smp_config_generic() is safe for pc-2.6 too.
it breaks migration as it could prevent target from starting if
there is hotplugged CPUs on source side.