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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 10/03/2017 04:56 AM, Roger Pau Monné wrote:
> On Fri, Sep 29, 2017 at 05:22:25PM +0000, Boris Ostrovsky wrote:
> > On 09/29/2017 01:07 PM, Roger Pau Monné wrote:
> > > On Fri, Sep 29, 2017 at 05:02:48PM +0000, Boris Ostrovsky wrote:
> > > > On 09/29/2017 11:51 AM, 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.
> > > > >
> > > > > Do you recall what's preventing the native entry point from booting
> > > > > inside of a PVH container? If certain emulated devices not present are
> > > > > needed at early boot we could look into either replacing them with
> > > > > other options available inside of a PVH container, or as a last resort
> > > > > making them available on a PVH container.
> > > > Very much IIRC one of the reasons was the fact that zeropage
> > > > (bootparams) needed to be properly formatted. And the hypercall page
> > > > needs to be set up.
> > > But in this case bootparams is going to be setup by grub, so it should
> > > be fine (just like it's done on bare metal).
> > Yes, I think so.
> >
> > > Couldn't the hypercall page be setup at some point during early boot?
> > > Not sure if setting it up at the same point HVM does would be fine for
> > > PVH?
> > Probably --- I think the only reason we set it early is because we need
> > to call XENMEM_memory_map to set bootparams. One other thing I noticed
> > is that we need to set acpi_irq_model before hypervisor is discovered
> > (can't remember why) but I suppose this can be worked around.
> >
> > Having said that --- since for direct boot we still need to go via
> > PVH-specific entry point I am not sure how much we will gain by having
> > grub avoid it.
> Being able to boot inside of a PVH container using the native entry
> point would prevent us from having to add PVH loader specific code to
> each firmware we plan to support in PVH mode.
>
> If Linux must be started using the PVH entry point in order to run
> inside of a PVH container, it means we would have to add a PVH loader
> to OVFM and grub at least.
>
> OTOH if Linux is capable of booting from the native entry point inside
> of a PVH container, we would only have to port OVMF and grub in order
> to work inside of a PVH container, leaving the rest of the logic
> untouched.
Right, I understand that. I was simply trying to say that PVH-specific 
entry point is likely to stay for us to continue booting PVH guests 
directly.
-boris

-boris

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