Re: [Xen-devel] Xen PVH support in grub2

[Boris Ostrovsky]

On 11/07/2017 02:42 AM, Juergen Gross wrote:
> On 06/11/17 17:42, Boris Ostrovsky wrote:
>> On 11/06/2017 10:05 AM, Juergen Gross wrote:
>>> On 06/11/17 15:51, Boris Ostrovsky wrote:
>>>> On 11/06/2017 02:16 AM, Juergen Gross wrote:
>>>>> On 03/11/17 20:00, Boris Ostrovsky wrote:
>>>>>> On 11/03/2017 02:40 PM, Juergen Gross wrote:
>>>>>>> On 03/11/17 19:35, Boris Ostrovsky wrote:
>>>>>>>> On 11/03/2017 02:23 PM, Juergen Gross wrote:
>>>>>>>>> On 03/11/17 19:19, Boris Ostrovsky wrote:
>>>>>>>>>> On 11/03/2017 02:05 PM, Juergen Gross wrote:
>>>>>>>>>>> So again the question: how to tell whether we are PVH or HVM in
>>>>>>>>>>> init_hypervisor_platform()? ACPi tables are scanned way later...
>>>>>>>>>> Can we make grub/OVMF append a boot option?
>>>>>>>>>>
>>>>>>>>>> Or set setup_header.hardware_subarch to something? We already have
>>>>>>>>>> X86_SUBARCH_XEN but it is only used by PV.  Or we might be able to 
>>>>>>>>>> use
>>>>>>>>>> hardware_subarch_data (will need to get a buy-in from x86 
>>>>>>>>>> maintainers, I
>>>>>>>>>> think).
>>>>>>>>> But wouldn't this break the idea to reuse the native boot paths in
>>>>>>>>> grub/OVMF without further modifications?
>>>>>>>> WDYM? We will have to have some sort of a plugin in either one to build
>>>>>>>> the zeropage anyway. So we'd set hardware_subarch there, in addition to
>>>>>>>> other things like setting memory and such.
>>>>>>> But isn't the zeropage already being built? I admit that setting subarch
>>>>>>> isn't a big deal, but using another entry with a passed-through pvh
>>>>>>> start struct isn't either...
>>>>>> I don't follow, sorry. My understanding is that zeropage will be built
>>>>>> by PVH-enlightened grub so part of this process would be setting the
>>>>>> subarch bit.
>>>>> My reasoning was based on Roger's remark:
>>>>>
>>>>> "OTOH if Linux is capable of booting from the native entry point inside
>>>>> of a PVH container, we would only have to port OVMF and grub in order
>>>>> to work inside of a PVH container, leaving the rest of the logic
>>>>> untouched."
>>>> Right, and in my mind porting OVMF/grub includes creating proper zeropage.
>>> Aah, okay. I reasoned on the assumption to just enable OVMF/grub to run
>>> in PVH environment without touching the parts setting up anything for
>>> the new kernel.
>> Someone needs to do what xen_prepare_pvh() does.
> As the loader is filling in the memory map information 

That's the thing that I thought may need to be done by us (setting
commandline too) . But I haven't looked at Xen support in grub so maybe
it's already there.

> the only thing
> remaining would be setting xen_pvh. And this could be delayed as my test
> have shown, so we only need to detect the PVH case from inside the
> kernel. One possibility would be the flags in the ACPI FADT table as
> Roger mentioned, another idea would be a flag in zeropage set by the
> loader.
>
>> And, for 64-bit, we also may need to build early pagetables since
>> startup_64() (unlike startup_32()) expects paging to be on. (I don't
>> know whether this is already part of standard FW codepath)
> This would be done the same way as for a native kernel.

This is done by Linux trampoline code. AFAIK grub loads kernel in realmode.


-boris