Re: [Qemu-devel] [PATCH 04/27] vl: convert -m to qemu_opts_parse()

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 27 Nov 2013 15:35:09 +0100
Markus Armbruster <armbru@redhat.com> wrote:

> Igor Mammedov <imammedo@redhat.com> writes:
> 
> > On Tue, 26 Nov 2013 15:49:05 +0100
> > Markus Armbruster <armbru@redhat.com> wrote:
> >
> >> Igor Mammedov <imammedo@redhat.com> writes:
> >> 
> >> > On Thu, 21 Nov 2013 11:12:43 +0100
> >> > Markus Armbruster <armbru@redhat.com> wrote:
> >> >
> >> >> Igor Mammedov <imammedo@redhat.com> writes:
> >> >> 
> > [...]
> >> Two separate issues here:
> >> 
> >> 1. The "no qemu_mem_opts have been specified" case
> >> 
> >>    This is equivalent to "empty options".  Therefore, the case can be
> >>    eliminated by pre-creating empty options.  No objection.
> >> 
> >>    The three existing merge_lists users don't do that.  Perhaps they
> >>    should.
> >> 
> >> 2. How to provide default values
> >> 
> >>    Supplying defaults is left to the caller of qemu_opt_get_FOO() by
> >>    design.
> >> 
> >>    Pre-creating option parameters deviates from that pattern.  You
> >>    justify it by saying it "eliminates need to pepper code with
> >>    DEFAULT_RAM_SIZE * 1024 * 1024".  How many occurrences?
> > beside of vl.c for:
> >   mem & maxmem 1 in hw/i386/pc.c
> >   slots - 6 in several files
> 
> Could the common code be factored out the old-fashioned way?
replacing one one-liner with another might help a little but
won't change a thing in general. It will be essentially the same.

> 
> Precedence: qemu_get_machine_opts() encapsulates some QemuOpts-related
> details, so its many users don't have to deal with them.
> 
> > see below for continuation:
> >
> >> 
> >>    Drawback: you lose the ability to see whether the user gave a value.
> >>    See below.
> >> 
> > [...]
> >> >> Ugly.
> >> >> 
> >> >> Why is the variable called 'end'?
> >> > would be 'suffix' better?
> >> 
> >> end points to the whole value string, not the end of anything, and
> >> neither to a suffix of anything.
> > Any suggestions?
> 
> What about val?
I've replaced it with "mem_str" see
"[PATCH 04/28] vl: convert -m to QemuOpts"

> 
> > [...]
> >> >> If you refrain from putting defaults into opts, you can distinguish the
> >> >> cases "user didn't specify maxmem, so assume mem", and "user specified
> >> >> maxmem, so check it's >= mem".
> >> > So foar, there is no point in distinguishing above cases,
> >> > since maxmem <= mem is invalid value and hotplug should be disabled.
> >> > So setting default maxmem to mem or anything less effectively
> >> > disables hotplug.
> >> 
> >> Yes, setting maxmem < mem is invalid and should be rejected, but not
> >> setting maxmem at all should be accepted even when you set mem.
> >> 
> >> Your patch like this pseudo-code:
> >> 
> >>     mem = DEFAULT_RAM_SIZE * 1024 * 1024
> >>     maxmem = mem
> >> 
> >>     if user specifies mem:
> >>         mem = user's mem
> >>     if user specifes max-mem:
> >>         mem = user's max-mem
> >> 
> >>     if max-mem < mem
> >>         what now?
> >>         should error our if max-mem and mem were specified by the user
> >>         shouldn't if user didn't specify max-mem!
> >>         but can't say whether he did
> >> 
> >> I'd do it this way:
> >> 
> >>     mem = unset
> >>     maxmem = unset
> >> 
> >>     if user specifies mem:
> >>         mem = user's mem
> >>     if user specifes max-mem:
> >>         mem = user's max-mem
> >> 
> >>     if mem != unset && max-mem != unset && max-mem < mem
> >>         error
> >>
> >> I'd use QemuOpts for the user's command line, and no more.  For anything
> >> beyond that, I'd use ordinary variables, such as ram_size.
> > Ok, I'll revert to the old code where options users check for option
> > availability, it's not that much code.
> >
> >
> > As for using QemuOpts as global store for global variables:
> >
> >  * using local variables would require changing of machine init or/and
> >    QEMUMachine and changing functions signature pass them down the stack to
> >    consumers.
> 
> Extending QEMUMachineInitArgs should suffice.  Once you're inside the
> board code, passing stuff around as C parameters is probably an
> improvement over passing around QemuOpts.
> 
> >  * adding "slots" readonly property to i440fx & q35 for consumption in
> >    ACPI hotplug code and building ACPI tables. It would be essentially another
> >    global lookup for i440fx & q35  object and pulling "slots" property,
> >    which is much longer way/complex way to get global value. That's a lot of
> >    boilerplate code for the same outcome.
> 
> Can't say without seeing the code.
> 
> >  * about setting default for "mem" value: if default "mem" is not set and
> >    no -m is provided on CLI, we get case where
> >       ram_size = foo & "mem" unset  
> >    And if I recall correctly there was an effort to provide interface for
> >    currently used QemuOpts to external users. So "mem" would get inconsistent
> >    with what QEMU uses.
> 
> QemuOpts do not record what QEMU uses.  They record what the user asked
> for.
> 
> > To sum up above said:
> >  * I'd like to continue using QemuOpts as global constant value store, it
> >    saves from adding a lot of boilerplate-code that would do the same.
> 
> Keeping the user's configuration just in QemuOpts is fine.  What I don't
> like is messing with it there.  This includes storing defaults.
> 
> Here's another reason: -writeconfig should write out exactly the user's
> configuration.  If you mess with it, it may write out messed up
> configuration, depending on *when* you mess with it.
> 
> >    Doing
> >      "git grep qemu_get_machine_opts"
> >    gets us several precedents that already use it that way.
> 
> Note that it does *not* store defaults in QemuOpts, it only creates
> empty opts.  I'm not sure that was a good idea.
I've dropped completely defaults setting in QemuOpts please see:
 "[PATCH 04/28] vl: convert -m to QemuOpts"
 "[PATCH 05/28] vl.c: extend -m option to support options for memory hotplug"

As for ">it only creates empty opts." I'm confused.
qemu_opt_get(qemu_get_machine_opts(), "foo")  pattern showed by grep  is the same
as I use to get "slots/maxmem":

exec.c:    if (!qemu_opt_get_bool(qemu_get_machine_opts(), "mem-merge", true)) {
hw/arm/boot.c:    info->dtb_filename = qemu_opt_get(qemu_get_machine_opts(), "dtb");
hw/ppc/spapr.c:    const char *drivename = qemu_opt_get(qemu_get_machine_opts(), "nvram");
hw/ppc/virtex_ml507.c:    dtb_filename = qemu_opt_get(qemu_get_machine_opts(), "dtb");
include/sysemu/sysemu.h:QemuOpts *qemu_get_machine_opts(void);
kvm-all.c:    if (!qemu_opt_get_bool(qemu_get_machine_opts(), "kernel_irqchip", true) ||
target-i386/kvm.c:    shadow_mem = qemu_opt_get_size(qemu_get_machine_opts(),

probably it is there because passing them as C parameters is more intrusive than 
just using user supplied values directly.

> >  * I believe that setting default in QemuOpts for "mem" is a good thing that
> >    leads to consistent meaning of "mem" with what QEMU actually uses.
> 
> I'm not sure I got this argument.
I can easily drop this hunk from "[PATCH 04/28] vl: convert -m to QemuOpts",
I've posted tonight as reply to this thread,
since ram_size is already passed to machine_init(), it's not worth arguing.