linux-kernel - Re: [Xen-devel] [PATCH 2/2] xen: add sysfs node for guest type

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Date:   Mon, 22 May 2017 14:33:20 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Juergen Gross <jgross@...e.com>, <linux-kernel@...r.kernel.org>,
        <xen-devel@...ts.xenproject.org>
CC:     <boris.ostrovsky@...cle.com>
Subject: Re: [Xen-devel] [PATCH 2/2] xen: add sysfs node for guest type

On 22/05/17 09:57, Juergen Gross wrote:
> Currently there is no reliable user interface inside a Xen guest to
> determine its type (e.g. HVM, PV or PVH). Instead of letting user mode
> try to determine this by various rather hacky mechanisms (parsing of
> boot messages before they are gone, trying to make use of known subtle
> differences in behavior of some instructions), add a sysfs node
> /sys/hypervisor/guest_type to explicitly deliver this information as
> it is known to the kernel.
>
> Signed-off-by: Juergen Gross <jgross@...e.com>
> ---
>  Documentation/ABI/testing/sysfs-hypervisor | 13 +++++++++++++
>  arch/arm/xen/enlighten.c                   |  3 +++
>  arch/x86/xen/enlighten.c                   |  3 +++
>  arch/x86/xen/enlighten_hvm.c               |  6 ++++--
>  arch/x86/xen/enlighten_pv.c                |  1 +
>  drivers/xen/sys-hypervisor.c               | 17 +++++++++++++++++
>  include/xen/xen.h                          |  2 ++
>  7 files changed, 43 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/ABI/testing/sysfs-hypervisor b/Documentation/ABI/testing/sysfs-hypervisor
> index 443196f0aa1c..06850f74ebd4 100644
> --- a/Documentation/ABI/testing/sysfs-hypervisor
> +++ b/Documentation/ABI/testing/sysfs-hypervisor
> @@ -19,6 +19,19 @@ Contact:	xen-devel@...ts.xenproject.org
>  Description:
>  		Compiler which was used to build the Xen hypervisor
>  
> +What:		/sys/hypervisor/guest_type
> +Date:		May 2017
> +KernelVersion:	4.12
> +Contact:	xen-devel@...ts.xenproject.org
> +Description:
> +		Type of guest:
> +		"native": standard guest type on arm
> +		"HVM": fully virtualized guest (x86)
> +		"PV": paravirtualized guest (x86)
> +		"PVH": fully virtualized guest without legacy emulation (x86)
> +		"PVHVM": fully virtualized guest using paravirtualized
> +			interfaces (e.g. interrupts, timers) (x86)

I'm not sure this is wise split.  PVHVM is a spectrum which changes
dynamically, especially in the presence of hardware APIC support.

I'd suggest guest type being straight PV or HVM (being the container
type), and a list of items (interrupts, timers, legacy emulation) which
are either using paravirt or native interfaces, or are not used at all.

~Andrew