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

[Markus Armbruster]

Igor Mammedov <address@hidden> writes:

> On Tue, 26 Nov 2013 15:49:05 +0100
> Markus Armbruster <address@hidden> wrote:
>
>> Igor Mammedov <address@hidden> writes:
>> 
>> > On Thu, 21 Nov 2013 11:12:43 +0100
>> > Markus Armbruster <address@hidden> wrote:
>> >
>> >> Igor Mammedov <address@hidden> 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?

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?

> [...]
>> >> 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 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.