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



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.

Roger.

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