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Re: [Qemu-devel] Turning off default storage devices?


From: Andy Lutomirski
Subject: Re: [Qemu-devel] Turning off default storage devices?
Date: Wed, 16 Apr 2014 08:40:32 -0700

On Mon, Apr 14, 2014 at 1:15 AM, Markus Armbruster <address@hidden> wrote:
> Peter Crosthwaite <address@hidden> writes:
>
>> Hi Andy,
>>
>> On Thu, Apr 10, 2014 at 5:55 AM, Andy Lutomirski <address@hidden> wrote:
>>> Currently, -M q35 boots linux quite a bit slower than the default
>>> machine type.  This seems to be because it takes a few hundred ms to
>>> determine that there's nothing attached to the AHCI controller.
>>>
>>> In virtio setups, there will probably never be anything attached to
>>> the AHCI controller.  Would it be possible to add something like
>>> -machine default_storage=off to turn off default storage devices?
>>> This could include the AHCI on q35 and the cdrom and such on pc.
>>>
>>> There's precedent: -machine usb=off turns off the default USB
>>> controllers, which is great for setups that use xhci.
>>>
>>
>> Is there a more generic solution to your problem? Can you implement
>> command line device removal in a non specific way and avoid having to
>> invent AHCI or even "storage" specific arguments. You could
>> considering bringing the xhci use case you mentioned under the same
>> umbrella.
>
> USB has always been off by default, at least for the boards I'm familiar
> with, due to the USB emulation's non-trivial CPU use.
>
> There's no such thing as a Q35 board without USB in the physical world.
> Can't stop us from making a virtual one, of course.
>
> Likewise, there's no such thing as a Q35 board without AHCI in the
> physical world, and again that can't stop us from making a virtual one.
>
> The difference to USB is that our q35 machines have always had AHCI even
> with -nodefaults.  You seem to propose adding a switch to disable AHCI,
> yet leave it enabled with -nodefaults.
>
> -nodefaults should give you a board with all the optional components
> suppressed.

Will this break libvirt, which may expect -nodefaults to still come
with an IDE bus?

>
> On the one hand, I'd rather not add exceptions to -nodefaults "give me
> the board with all its optional components suppressed" semantics.
>
> On the other hand, a few hundred ms are a long time.

That's why I proposed a new option.  Yes, it's ugly :/

--Andy



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