Re: [libvirt] [Qemu-devel] [PATCH v2] vl.c: Support multiple CPU ranges on -numa option
Anthony Liguori <anthony(a)codemonkey.ws> writes:
Paolo Bonzini <pbonzini(a)redhat.com> writes:
> Il 27/02/2013 18:08, Anthony Liguori ha scritto:
>>> >
>>> > No, no, no. This makes ':' special, which means you can't
have lists of
>>> > anything containing ':'. Your cure is worse than the disease.
Let go
>>> > of that syntactic high-fructose corn syrup, stick to what we have and
>>> > works just fine, thank you.
>> Yes, there *must* be special syntax. If we're treating something
>> special, then we should indicate to the user that it's special.
>>
>> Specifically, a list of integers should look distinctly different than
>> overriding a previously specified integer.
>
> The solution is "there is no way to override a previously specified
> key". Something like "-device
> virtio-scsi-pci,num_queues=1,num_queues=2" now works, let's make it an
> error instead.
That breaks compatibility. The above may seem silly but consider:
qemu -device virtio-scsi-pci,num_queues=1,id=foo \
-set device.foo.num_queues=2
This is more common than you would think primarily as a way to override
options that libvirt has set either via the qemu extra args tag or a
script wrapper of qemu.
Related: overwrite something you got from a config file on the command
line.
In both your example and mine, we have entirely separate options, and
they have perfectly ordinary overwrite semantics: each option overwrites
the given keys with the given values. The last key=value wins.
This usage makes sense, and we obviously want to preserve it.
Paolo's example is different only in that it's a silly. Preserving
compatibility may mean that once we accepted silly usage, we can't ever
reject it. Debatable. Personally, I disagree: I think we could outlaw
repeating keys within the same option argument / configuration file
section just fine.
Finally, I don't think that we must have fancy-pants syntax to remind
users that they're configuring a list. What's the chance of confusion
there? What user would expect num_queues=1,num_queues=2 to make
num_queues magically become a list?