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

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Date:   Wed, 08 Nov 2017 05:31:18 -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 12:55, <jgross@...e.com> wrote:
> 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.

My point is that "legacy free" would likely be reached over time (and
even once fully reached, hybrid configurations would be possible).
I.e. there could be a setup with PIC, but with neither VGA nor RTC.
That's still not PVH then. Nor do all legacy features require a device
model in the first place - some of them are being emulated entirely
in the hypervisor.

Furthermore, PVH absolutely requires guest awareness afaict, while
legacy-free pure HVM guests (with an OS only aware of the possible
absence of legacy devices) would still be possible.

Jan