Re: [libvirt] [Qemu-devel] [PATCH v5 1/2] vl.c deprecate incorrect CPUs topology

On Thu, Sep 06, 2018 at 10:02:13AM +0200, Igor Mammedov wrote: > On Wed, 5 Sep 2018 10:45:12 -0300 > Eduardo Habkost <ehabkost(a)redhat.com&gt; wrote: > > > On Wed, Sep 05, 2018 at 11:25:11AM +0200, Igor Mammedov wrote: > > > On Tue, 4 Sep 2018 23:12:55 -0300 > > > Eduardo Habkost <ehabkost(a)redhat.com&gt; wrote: > > > > > > > On Tue, Sep 04, 2018 at 03:22:35PM +0200, Igor Mammedov wrote: > > > > > -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: Igor Mammedov <imammedo(a)redhat.com&gt; > > > > > --- > > > > > v5: > > > > > - extend deprecation doc, adding that maxcpus should be multiple of > > > > > present on CLI [sockets/cores/threads] options > > > > > (Eduardo Habkost <ehabkost(a)redhat.com>) > > > > > v4: > > > > > - missed dot comment, fix it with s/./,/ (Andrew Jones <drjones(a)redhat.com>) > > > > > v3: > > > > > - more spelling fixes (Andrew Jones <drjones(a)redhat.com>) > > > > > - place deprecation check after (sockets * cores * threads > max_cpus) check > > > > > (Eduardo Habkost <ehabkost(a)redhat.com>) > > > > > v2: > > > > > - spelling&&co fixes (Andrew Jones <drjones(a)redhat.com>) > > > > > --- > > > > > qemu-deprecated.texi | 19 +++++++++++++++++++ > > > > > vl.c | 7 +++++++ > > > > > 2 files changed, 26 insertions(+) > > > > > > > > > > diff --git a/qemu-deprecated.texi b/qemu-deprecated.texi > > > > > index 87212b6..827c3ce 100644 > > > > > --- a/qemu-deprecated.texi > > > > > +++ b/qemu-deprecated.texi > > > > > @@ -159,6 +159,25 @@ The 'file' driver for drives is no longer appropriate for character or host > > > > > devices and will only accept regular files (S_IFREG). The correct driver > > > > > for these file types is 'host_cdrom' or 'host_device' as appropriate. > > > > > > > > > > +@subsection -smp X,[socket=a,core=b,thread=c],maxcpus=Y (since 3.1) > > > > > > > > Minor: I suggest using @var markup, or maybe just use > > > > "-smp (invalid topologies) (since 3.1)" as subsection title for simplicity? > > > > > > > > > + > > > > > +CPU topology properties should describe whole machine topology including > > > > > +possible CPUs, but historically it was possible to start QEMU with > > > > > +an incorrect topology where > > > > > + sockets * cores * threads >= X && X < maxcpus > > > > > > > > Minor: this line formatting is lost on the HTML output. I > > > > suggest using @var and/or @math. > > > > > > > > Minor: I suggest not using C syntax. > > > > > > > > i.e. use something like: > > > > > > > > @math{@var{n} <= @var{sockets} * @var{cores} * @var{threads} < @var{maxcpus}}, > > > > > > > > > +which could lead to an incorrect topology enumeration by the guest. > > > > > +Support for invalid topologies will be removed, the user must ensure > > > > > +topologies described with -smp include all possible cpus, i.e. > > > > > + sockets * cores * threads == maxcpus > > > > > > > > Minor: same as above. I suggest: > > > > > > > > @math{@var{sockets} * @var{cores} * @var{threads} = @var{maxcpus}}. > > > > > > > > > > > > > +Note: it's assumed that maxcpus value must be multiple of the topology > > > > > +options present on command line to avoid creating an invalid topology. > > > > > +If maxcpus isn't be multiple of present topology options then the condition > > > > > +(sockets * cores * threads == maxcpus) can't be satisfied and it will > > > > > +trigger deprecation warning which later will be converted to a error. > > > > > +If you get deprecation warning it's recommended to explicitly specify > > > > > +a correct topology to make warning go away and ensure that it will > > > > > +continue working in the future. > > > > > > > > I don't understand the purpose of the "Note:" section. It seems to duplicate > > > > what was already said in the lines above it. Can we just remove it? > > > Well, didn't I say that no additional explanation needed in v4? > > > Note section was added per your suggestion to explicitly say that maxcpus > > > should be multiple of options present on CLI. > > > > You are right: I did ask for additional clarification on the > > documentation, because it was not clear that the warning can > > still appear even if some of the sockets/cores/threads options > > were omitted. The note didn't make that clearer to me, though. > I can respin if necessary (we are not in hurry so no need to merge > something that would be removed immediately). > Could you suggest a text for clarification for me to include on respin? I was planning to write a suggestion later, but I didn't want this series to be delayed because of me. Just removing the existing "Notes:" section would be good enough to me. Probably the right place to document the subtle details of the s/c/t/maxcpus requirements is the "-smp" section on qemu-options.hx, not qemu-deprecated.hx. I was considering something like this: Signed-off-by: Eduardo Habkost <ehabkost(a)redhat.com&gt; --- diff --git a/qemu-deprecated.texi b/qemu-deprecated.texi index 060e015be6..74f6a64b8b 100644 --- a/qemu-deprecated.texi +++ b/qemu-deprecated.texi @@ -139,16 +139,20 @@ The 'file' driver for drives is no longer appropriate for character or host devices and will only accept regular files (S_IFREG). The correct driver for these file types is 'host_cdrom' or 'host_device' as appropriate. -@subsection -smp X,[socket=a,core=b,thread=c],maxcpus=Y (since 3.1) +@subsection -smp (invalid topologies) (since 3.1) CPU topology properties should describe whole machine topology including -possible CPUs, but historically it was possible to start QEMU with +possible CPUs. In other words: @var{maxcpus} should be equal to +@math{@var{sockets} * @var{cores} * @var{threads}}. + +However, historically it was possible to start QEMU with an incorrect topology where - sockets * cores * threads >= X && X < maxcpus +@math{@var{n} <= @var{sockets} * @var{cores} * @var{threads} < @var{maxcpus}}, which could lead to an incorrect topology enumeration by the guest. Support for invalid topologies will be removed, the user must ensure topologies described with -smp include all possible cpus, i.e. - sockets * cores * threads == maxcpus +@math{@var{sockets} * @var{cores} * @var{threads} = @var{maxcpus}}. +

@section QEMU Machine Protocol (QMP) commands diff --git a/qemu-options.hx b/qemu-options.hx index 654ef484d9..2afa271570 100644 --- a/qemu-options.hx +++ b/qemu-options.hx @@ -155,8 +155,13 @@ to 4. For the PC target, the number of @var{cores} per socket, the number of @var{threads} per cores and the total number of @var{sockets} can be specified. Missing values will be computed. If any on the three values is -given, the total number of CPUs @var{n} can be omitted. @var{maxcpus} -specifies the maximum number of hotpluggable CPUs. +given, the total number of CPUs @var{n} can be omitted. + +@var{maxcpus} specifies the maximum number of hotpluggable CPUs +and should be equal to @math{@var{sockets} * @var{cores} * @var{threads}} +(even if @var{sockets}, @var{cores}, or @var{threads} is omitted +and computed automatically).

ETEXI DEF("numa", HAS_ARG, QEMU_OPTION_numa,