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

From: Boris Ostrovsky
Subject: Re: [Xen-devel] Xen PVH support in grub2
Date: Fri, 29 Sep 2017 13:22:25 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0

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.


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