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

[Juergen Gross <jgross@xxxxxxxx>]

On 03/11/17 16:10, Boris Ostrovsky wrote:
> On 11/03/2017 10:59 AM, Juergen Gross wrote:
>> On 03/11/17 15:36, Boris Ostrovsky wrote:
>>> On 11/03/2017 10:24 AM, Juergen Gross wrote:
>>>> On 03/11/17 15:07, Roger Pau Monné wrote:
>>>>> On Fri, Nov 03, 2017 at 01:50:11PM +0100, Juergen Gross wrote:
>>>>>> On 03/11/17 13:17, Roger Pau Monné wrote:
>>>>>>> On Fri, Nov 03, 2017 at 01:00:46PM +0100, Juergen Gross wrote:
>>>>>>>> On 29/09/17 17:51, Roger Pau Monné wrote:
>>>>>>>>> On Fri, Sep 29, 2017 at 03:33:58PM +0000, Juergen Gross wrote:
>>>>>>>>>> On 29/09/17 17:24, Roger Pau Monné wrote:
>>>>>>>>>>> On Fri, Sep 29, 2017 at 02:46:53PM +0000, Juergen Gross wrote:
>>>>>>>>>>> Then, I also wonder whether it would make sense for this grub to 
>>>>>>>>>>> load
>>>>>>>>>>> the kernel using the PVH entry point or the native entry point. 
>>>>>>>>>>> Would
>>>>>>>>>>> it be possible to boot a Linux kernel up to the point where cpuid 
>>>>>>>>>>> can
>>>>>>>>>>> be used inside of a PVH container?
>>>>>>>>>> I don't think today's Linux allows that. This has been discussed
>>>>>>>>>> very thoroughly at the time Boris added PVH V2 support to the kernel.
>>>>>>>>> OK, I'm not going to insist on that, but my plans for FreeBSD is to
>>>>>>>>> make the native entry point capable of booting inside of a PVH
>>>>>>>>> container up to the point where cpuid (or whatever method) can be used
>>>>>>>>> to detect the environment.
>>>>>>>> Looking more thoroughly into the Linux boot code I think this could
>>>>>>>> work for Linux, too. But only if we can tell PVH from HVM in the guest.
>>>>>>>> How would you do that in FreeBSD? Via flags in the boot params? This
>>>>>>>> would the have to be done in the boot loader (e.g. grub or OVMF).
>>>>>>> My plan was not to differentiate between HVM and PVH, but rather to
>>>>>>> make use of the ACPI information in order to decide which devices are
>>>>>>> available and which are not inside of a PVH guest.
>>>>>>>
>>>>>>> For example in the FADT "IA-PC Boot Architecture Flags" field for PVH
>>>>>>> we already set "VGA Not Present" and "CMOS RTC Not Present". There
>>>>>>> might be other flags/fields that must be set, but I would like to
>>>>>>> avoid having a CPUID bit or similar saying "PVH", because then Xen
>>>>>>> will be tied to always providing the same set of devices in PVH
>>>>>>> containers.
>>>>>> Why? This would depend on the semantics tied to the flag. It could just
>>>>>> mean "don't assume availability of legacy stuff" (e.g. BIOS calls).
>>>>>>
>>>>>> Linux would have a problem with the ACPI approach as it would try BIOS
>>>>>> calls way before it is initializing its ACPI handling. So in Linux I'd
>>>>>> need another way to tell I'm running in PVH mode, e.g. a "no legacy"
>>>>>> bit in the Xen HVM cpuid leaf.
>>>>> If you are booted from the PVH entry point, there's no BIOS or UEFI
>>>>> (ie: no firmware), if you are booted from the BIOS entry point there's
>>>>> a BIOS and the same applies to UEFI. How does Linux differentiate
>>>>> whether it's booted from BIOS or UEFI?
>>>> They use different entries.
>>> In fact, we had a discussion with Matt Fleming (Linux EFI maintainer) to
>>> see if we can use EFI entry point to also be able to boot PVH guest but
>>> found some issues with that approach, which is why we ended up with a
>>> dedicated PVH entry point.
>>>
>>> I am curious though, Juergen --- what do we need besides zeropage to
>>> allow us to boot PVH from startup_64?
>> Oh, you are right. I managed to get lost in the early boot paths.
>>
>> Only setting up the hyperpage seems to be missing, but this should be
>> doable. And setting xen_pvh, of course.
> 
> That last part was actually my question --- do we need to have xen_pvh
> set before we get to xen-specific code for the first time (which I think
> is init_hypervisor_platform()) from startup_64?
> 
> Because if we do --- who will set it?

This should be easily testable: Just set an internal indicator in
xen_prepare_pvh() and copy that to xen_pvh in init_hypervisor_platform()
and see whether the kernel still comes up in PVH mode.

The same can be done with setting up the hypercall page.

Just building a kernel trying that...


Juergen


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