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

[Juergen Gross <jgross@xxxxxxxx>]

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.


Juergen

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