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

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Message-ID: <256b90df-547e-3287-d13f-3ffb346b4bc2@redhat.com>
Date:   Wed, 8 Nov 2017 13:03:44 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Juergen Gross <jgross@...e.com>, 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, 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 08/11/2017 12:55, Juergen Gross 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 understanding of Xen is very rusty at this point, but I think a
"completely" legacy-free HVM domain will still have a PCI bus and the
Xen platform device on that bus.

A PVH domain just knows how to access the Xen PV features.

Paolo