[Qemu-devel] Turning off default storage devices?

[Qemu-devel] Turning off default storage devices?

[Andy Lutomirski]

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.

Thanks,
Andy

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

[Peter Crosthwaite]

Hi Andy,

On Thu, Apr 10, 2014 at 5:55 AM, Andy Lutomirski <luto@amacapital.net> 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.

Regards,
Peter

> Thanks,
> Andy
>

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

[Andy Lutomirski]

On Wed, Apr 9, 2014 at 4:53 PM, Peter Crosthwaite
<peter.crosthwaite@xilinx.com> wrote:
> Hi Andy,
>
> On Thu, Apr 10, 2014 at 5:55 AM, Andy Lutomirski <luto@amacapital.net> 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.

An option like -suppress-default-device foobar to turn off the device
named foobar would work, but what happens if that device is a bus?
Will this just cause QEMU to crash?  Maybe the machine code would have
to opt in to allowing this kind of suppression, and there could be a
general error of you try to suppress a device that can't be
suppressed.

I can try to code this up, but I know nothing about QEMU internals.
I'm just a user :)

--Andy

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

[Peter Crosthwaite]

On Thu, Apr 10, 2014 at 9:57 AM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Wed, Apr 9, 2014 at 4:53 PM, Peter Crosthwaite
> <peter.crosthwaite@xilinx.com> wrote:
>> Hi Andy,
>>
>> On Thu, Apr 10, 2014 at 5:55 AM, Andy Lutomirski <luto@amacapital.net> 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.
>
> An option like -suppress-default-device foobar to turn off the device
> named foobar would work, but what happens if that device is a bus?

Lets call that a misuse in the first instance. But in general, when
attaching devices QEMU should be able to gracefully fail on unresolved
deps. So it would be reasonable to work on that assumption given that
every device should be able to handle a missing bus/gpio/interrupt
etc. due to -device misuseability.

> Will this just cause QEMU to crash?  Maybe the machine code would have
> to opt in to allowing this kind of suppression, and there could be a
> general error of you try to suppress a device that can't be
> suppressed.
>

I would argue that there is no such thing. You may end up with a
useless machine but its still valid to supress something and then by
extension all its dependants are non functional.

Regards.
Peter

> I can try to code this up, but I know nothing about QEMU internals.
> I'm just a user :)
>
> --Andy
>