Re: [Qemu-devel] [Qemu-ppc] Qemu boot device precedence over nvram boot-device setting

[Avik Sil <aviksil@linux.vnet.ibm.com>]

On 10/04/2012 04:52 PM, Gleb Natapov wrote:
> On Thu, Oct 04, 2012 at 04:25:28PM +0530, Avik Sil wrote:
>> On 09/27/2012 03:21 PM, Gleb Natapov wrote:
>>> On Thu, Sep 27, 2012 at 11:33:31AM +0200, Alexander Graf wrote:
>>>>
>>>> On 27.09.2012, at 11:29, Benjamin Herrenschmidt wrote:
>>>>
>>>>> On Thu, 2012-09-27 at 14:51 +0530, Avik Sil wrote:
>>>>>> Hi,
>>>>>>
>>>>>> We would like to get a method to boot from devices provided in -boot
>>>>>> arguments in qemu when the 'boot-device' is set in nvram for pseries
>>>>>> machine. I mean the boot device specified in -boot should get a
>>>>>> precedence over the 'boot-device' specified in nvram.
>>>>>>
>>>>>> At the same time, when -boot is not provided, i.e., the default boot
>>>>>> order "cad" is present, the device specified in nvram 'boot-device'
>>>>>> should get precedence if it is set.
>>>>>>
>>>>>> What should be the elegant way to implement this requirement?
>>>>>> Suggestions welcome.
>>>>>
>>>>> Actually I think it's a more open question. We have essentially two
>>>>> things at play here:
>>>>>
>>>>> - With the new nvram model, the firmware can store a boot device
>>>>> reference in it, which is standard OF practice, and in fact the various
>>>>> distro installers are going to do just that
>>>>>
>>>>> - Qemu has its own boot order thingy via -boot, which we loosely
>>>>> translate as c = first bootable disk we find (actually first disk we
>>>>> find, we should probably make the algorithm a bit smarter), d = first
>>>>> cdrom we find, n = network , ... We pass that selection (boot list) down
>>>>> to SLOF via a device-tree property.
>>>>>
>>>>> The question is thus what precedence should we give them. I was
>>>>> initially thinking that an explicit qemu boot list should override the
>>>>> firmware nvram setting but I'm now not that sure anymore.
>>>>>
>>>>> The -boot list is at best a "blurry" indication of what type of device
>>>>> the user wants ... The firmware setting in nvram is precise.
>>>>
>>>> IIRC gleb had implemented a specific boot order thing. Gleb, mind to enlighten us? :)
>>>>
>>> Yes, forget about -boot. It is deprecated :) You should use bootindex
>>> (device property) to set boot priority. It constructs OF device path
>>> and passes it to firmware. There is nothing "blurry" about OF device
>>> path. The problem is that it works reasonably well with legacy BIOS
>>> since it is enough to specify device to boot from, but with EFI (OF is
>>> the same I guess) it is not enough to point to a device to boot from,
>>> but you also need to specify a file you want to boot and this is where
>>> bootindex approach fails. If EFI would specify default file to boot from
>>> firmware could have used it, but EFI specifies it only for removable media
>>> (what media is not removable this days, especially with virtualization?).
>>> We can add qemu parameter to specify file to boot, but how users should
>>> know the name of the file?
>>>
>> I looked at the bootindex stuff and found that when the bootindex is
>> specified for the disk and cdrom it generates a string like:
>>
>> "/spapr-vio-bridge/spapr-vscsi/channel@0/disk@0,1
>> /spapr-vio-bridge/spapr-vscsi/channel@0/disk@0,0"
>>
>> Now converting/translating this to OF device path is going to be
>> much trickier and might not be proper. So I propose a simple
>> solution by introducing a global flag that checks if explicit -boot
>> parameter is provided or not. The presence of this parameter is
>> verified in SLOF firmware. The flag had to be introduced as
>> boot_devices defaults to "cad" instead of null and passed to
>> machine->init().
>>
> So you want to hack around the problem. If -boot is specified what
> device are you going to boot from?

It is going to boot from the device specified in -boot as 
default_boot_order is set to 0 in that case.

Regards,
Avik