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

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Message-Id: <5A02F633020000780018D26A@prv-mh.provo.novell.com>
Date:   Wed, 08 Nov 2017 04:18:58 -0700
From:   "Jan Beulich" <JBeulich@...e.com>
To:     "Juergen Gross" <jgross@...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 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.

Jan