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

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Date:   Mon, 22 May 2017 16:16:36 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Andrew Cooper <andrew.cooper3@...rix.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 15:33, Andrew Cooper wrote:
> 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.

Dropping PVHVM from this list is okay, but I'd like to keep PVH. Even if
technically it _is_ HVM without legacy emulation, for the user it is
more some kind of a guest type than just an attribute.


Juergen