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Re: [libvirt] [Qemu-devel] [PATCH] vl.c: Support multiple CPU ranges on -numa option



Eduardo Habkost <ehabkost redhat com> writes:

> On Thu, Feb 21, 2013 at 09:23:22PM +0100, Markus Armbruster wrote:
>> Eduardo Habkost <ehabkost redhat com> writes:
>> 
>> > This allows "," to be used a separator between each CPU range.  Note
>> > that commas inside key=value command-line options have to be escaped
>> > using ",,", so the command-line will look like:
>> >
>> >   -numa node,cpus=A,,B,,C,,D
>> 
>> This is really, really ugly, and an embarrassment to document.  Which
>> you didn't ;)
>
> I was trying to have an intermediate solution using the current -numa
> parser. I have patches in my queue that will change the code to properly
> use QemuOpts later.
>
> It would be interesting to support the "A,B,C,D" format in config files,
> though, as it is simple and straighforward when no escaping is required.

Our config file syntax is in a Windows INI dialect: key=value lines
grouped into sections.  Our dialect requires values to be enclosed in
quotes.  Commonly, the quotes are optional.  Could be fixed.  It
supports multi-valued keys the common INI way: multiple key=value lines
for the same key, one per value

key = "A,B,C" works when the A, B, C can't contain commas.  Fine for a
list of numbers.  For long lists, we'd probably want to add a line
continuation feature.

Strings can contain commas, so you'd have to do something like key =
"A", "B", "C".  Whether that's still Windows INI is debatable.  More so
since there's already a common way to do it: one line per value.

If we decide INI doesn't meet our needs or desires for pretty syntax, we
should not extend it beyond its limits into QEMU's very own
configuration syntax.  We should switch to a common syntax that serves
our needs and desires.  For what it's worth, we already parse JSON.

For me, the INI way to do multi-valued keys is still fine.

>> What about
>> 
>>     -numa node,cpus=A,cpus=B,cpus=C,cpus=D
>
> Looks better for the command-line usage, at least. I will give it a try.
>
>> 
>> Yes, QemuOpts lets you do that. Getting all the values isn't as easy as
>> it could be (unless you use Laszlo's opt-visitor), but that could be
>> improved.
>
> Guess what: -numa doesn't even use QemuOpts, and I am not sure the
> current format of -numa will allow QemuOpts to be used easily. I expect
> the proper solution using QemuOpts to involve having a
> standards-compliant "numa-node" config section instead of this weird
> "-numa <type>,..." format where the only valid <type> that ever existed
> was "node".

This is the current -numa syntax, as far as I can tell:

    -numa node,KEY=VALUE,...

Recognized KEY=VALUE:

    nodeid=UINT
    mem=SIZE
    cpus=[|UINT|UINT-UINT]

Unrecognized KEYs are silently ignored.

This should fit into QemuOpts just fine.  Sketch:

static QemuOptsList qemu_numa_opts = {
    .name = "numa",
    .implied_opt_name = "type"
    .head = QTAILQ_HEAD_INITIALIZER(qemu_rtc_numa.head),
    .desc = {
        {
            .name = "type",
            .type = QEMU_OPT_STRING,
            .help = "node type"
        }, {
            .name = "nodeid",
            .type = QEMU_OPT_NUMBER,
            .help = "node ID"
        }, {
            .name = "mem",
            .type = QEMU_OPT_SIZE,
            .help = "memory size"
        }, {
            .name = "cpus",
            .type = QEMU_OPT_STRING,
            .help = "CPU range"
        },
        { /* end of list */ }
    },
};


    type = qemu_opt_get(opts);
    if (!type || strcmp(type, "node)) {
        // error
    }
    // get and record nodeid, mem
    // get, parse and record cpus

This rejects unrecognized keys, unlike the current code.  Declare bug
fix ;)

To support discontinuous CPU sets, simply get all values of key "cpus".

> But I believe it will be feasible to allow "cpus=A,cpus=B" using the
> current parser, before we convert to a proper QemuOpts-based
> implementaiton.
>
>> 
>> > Note that the following format, currently used by libvirt:
>> >
>> >   -numa nodes,cpus=A,B,C,D
>> >
>> > will _not_ work yet, as "," is the option separator for the command-line
>> > option parser, and it will require changing the -numa option parsing
>> > code to handle "cpus" as a special case.
>> 
>> No way.
>
> Agreed.  :-)
>
> The bad news is that libvirt uses this format since forever, this format
> never worked, and nobody ever noticed that this was broken.

The good news is that it never worked, which simplifies our backward
compatibility worries.


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