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

[Juergen Gross <jgross@xxxxxxxx>]

On 03/11/17 16:27, Juergen Gross wrote:
> 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...

Okay, setting xen_pvh late is working. I can't omit setting up the
hypercall_page right now, of course, as then I wouldn't get the memory
map. But this shouldn't be a problem for the grub/OVMF case as those
would setup the memory map for the guest in zeropage.

So again the question: how to tell whether we are PVH or HVM in
init_hypervisor_platform()? ACPi tables are scanned way later...


Juergen

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel