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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

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?

Roger.

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