linux-kernel - Re: [Xen-devel] [PATCH 3/3] x86/xen: use guest_late

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Date:   Wed, 8 Nov 2017 12:55:11 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Jan Beulich <JBeulich@...e.com>
Cc:     len.brown@...el.com, x86@...nel.org, tglx@...utronix.de,
        xen-devel@...ts.xenproject.org, boris.ostrovsky@...cle.com,
        mingo@...hat.com, pbonzini@...hat.com, rkrcmar@...hat.com,
        rjw@...ysocki.net, pavel@....cz, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, hpa@...or.com
Subject: Re: [Xen-devel] [PATCH 3/3] x86/xen: use guest_late_init to detect
 Xen PVH guest

On 08/11/17 12:18, Jan Beulich wrote:
>>>> On 08.11.17 at 10:07, <jgross@...e.com> wrote:
>> In case we are booted via the default boot entry by a generic loader
>> like grub or OVMF it is necessary to distinguish between a HVM guest
>> with a device model supporting legacy devices and a PVH guest without
>> device model.
>>
>> PVH guests will always have x86_platform.legacy.no_vga set and
>> x86_platform.legacy.rtc cleared, while both won't be true for HVM
>> guests.
>>
>> Test for both conditions in the guest_late_init hook and set xen_pvh
>> to true if they are met.
> 
> This sounds pretty fragile to me: I can't see a reason why a proper
> HVM guest couldn't come without VGA and RTC. That's not possible
> today, agreed, but certainly an option down the road if virtualization
> follows bare metal's road towards being legacy free.

>From guest's perspective: what is the difference between a legacy free
HVM domain and PVH? In the end the need for differentiating is to avoid
access to legacy features in PVH as those would require a device model.


Juergen