Re: [Minios-devel] [Xen-devel] [PATCH 1/2] mini-os: support keeping start_info structure conditionally

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 29/08/2016 13:28, Juergen Gross wrote:
> On 29/08/16 13:47, Andrew Cooper wrote:
>> On 29/08/2016 12:17, Juergen Gross wrote:
>>> On 29/08/16 13:09, Wei Liu wrote:
>>>> On Mon, Aug 29, 2016 at 01:01:08PM +0200, Juergen Gross wrote:
>>>>> grub stubdom needs the start_info structure. Keep a copy of it in
>>>>> pv mini-os if configured via "CONFIG_KEEP_STARTINFO". This should
>>>>> default to "n" in order to have it enabled only when really needed.
>>>>>
>>>> I'm not sure I understand the rationale for this.
>>>>
>>>> Shouldn't start_info always be kept when mini-os is PV? Under what
>>>> condition should it not be kept?
>>> The application on top of Mini-OS shouldn't depend on Mini-OS being
>>> paravirtualized or not in the "normal" case. grub-stubdom is a
>>> special case, as it needs to kexec to a loaded kernel which in turn
>>> needs the start_info, of course.
>>>
>>> ioemu-stubdom OTOH should not need start_info as it could work on
>>> a HVMlite Mini-OS, too.
>>>
>>> The idea of removing start_info in my HVMlite series was driven by
>>> that thought: any application using start_info should break as it
>>> clearly wouldn't be agnostic of the mode (pv or HVMlite) of Mini-OS.
>>> pv-grub was an oversight here.
>>>
>>> I'm planning to modify ioemu-stubdom in the future to not use
>>> start_info and then let it run in HVMlite mode, too.
>> There is an issue here between MiniOS itself, and "the stubdom application".
> Correct.
>
>> There should be no circumstance where the stubdom application needs
>> access (pv-grub being a special case, but maybe the kexec details can be
>> hidden as well).
> Indeed. I'll have a look if this is possible. In case I find a clean
> solution I'll send patches including one removing CONFIG_KEEP_STARTINFO
> again.
>
>> However, while there is still relevant information in start_info, the
>> low level PV bits should still have access.  Moreover, it is necessary
>> to always have access when doing suspend/resume.
> The information from start_info is available inside Mini-OS. So this
> should be no problem.

I have never understood MiniOS's decision to memcpy() it elsewhere.  It
is just a plain page of RAM set up by the domain builder; copying it
elsewhere is just a waste of space.

OTOH, you must pass a pointer to it in the suspend() hypercall, so the
resume logic can re-modify part of it.

~Andrew

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