linux-kernel - Re: [PATCH v2] drivers/virt: vmgenid: add vm generation id driver

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Date:   Thu, 19 Nov 2020 14:09:58 +0100
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Alexander Graf <graf@...zon.de>,
        "Catangiu, Adrian Costin" <acatan@...zon.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Jann Horn <jannh@...gle.com>
Cc:     Willy Tarreau <w@....eu>,
        "MacCarthaigh, Colm" <colmmacc@...zon.com>,
        Andy Lutomirski <luto@...nel.org>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Eric Biggers <ebiggers@...nel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        "Woodhouse, David" <dwmw@...zon.co.uk>,
        "bonzini@....org" <bonzini@....org>,
        "Singh, Balbir" <sblbir@...zon.com>,
        "Weiss, Radu" <raduweis@...zon.com>,
        "oridgar@...il.com" <oridgar@...il.com>,
        "ghammer@...hat.com" <ghammer@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Qemu Developers <qemu-devel@...gnu.org>,
        KVM list <kvm@...r.kernel.org>,
        Michal Hocko <mhocko@...nel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Pavel Machek <pavel@....cz>,
        Linux API <linux-api@...r.kernel.org>,
        "mpe@...erman.id.au" <mpe@...erman.id.au>,
        linux-s390 <linux-s390@...r.kernel.org>,
        "areber@...hat.com" <areber@...hat.com>,
        Pavel Emelyanov <ovzxemul@...il.com>,
        Andrey Vagin <avagin@...il.com>,
        Mike Rapoport <rppt@...nel.org>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Pavel Tikhomirov <ptikhomirov@...tuozzo.com>,
        "gil@...l.com" <gil@...l.com>,
        "asmehra@...hat.com" <asmehra@...hat.com>,
        "dgunigun@...hat.com" <dgunigun@...hat.com>,
        "vijaysun@...ibm.com" <vijaysun@...ibm.com>
Subject: Re: [PATCH v2] drivers/virt: vmgenid: add vm generation id driver



On 19.11.20 13:51, Alexander Graf wrote:
> 
> 
> On 19.11.20 13:02, Christian Borntraeger wrote:
>>
>> On 16.11.20 16:34, Catangiu, Adrian Costin wrote:
>>> - Background
>>>
>>> The VM Generation ID is a feature defined by Microsoft (paper:
>>> http://go.microsoft.com/fwlink/?LinkId=260709) and supported by
>>> multiple hypervisor vendors.
>>>
>>> The feature is required in virtualized environments by apps that work
>>> with local copies/caches of world-unique data such as random values,
>>> uuids, monotonically increasing counters, etc.
>>> Such apps can be negatively affected by VM snapshotting when the VM
>>> is either cloned or returned to an earlier point in time.
>>>
>>> The VM Generation ID is a simple concept meant to alleviate the issue
>>> by providing a unique ID that changes each time the VM is restored
>>> from a snapshot. The hw provided UUID value can be used to
>>> differentiate between VMs or different generations of the same VM.
>>>
>>> - Problem
>>>
>>> The VM Generation ID is exposed through an ACPI device by multiple
>>> hypervisor vendors but neither the vendors or upstream Linux have no
>>> default driver for it leaving users to fend for themselves.
>>
>> I see that the qemu implementation is still under discussion. What is
> 
> Uh, the ACPI Vmgenid device emulation is in QEMU since 2.9.0 :).

Ah right. Found it. 
> 
>> the status of the other existing implementations. Do they already exist?
>> In other words is ACPI a given?
>> I think the majority of this driver could be used with just a different
>> backend for platforms without ACPI so in any case we could factor out
>> the backend (acpi, virtio, whatever) but if we are open we could maybe
>> start with something else.
> 
> I agree 100%. I don't think we really need a new framework in the kernel for that. We can just have for example an s390x specific driver that also provides the same notification mechanism through a device node that is also named "/dev/vmgenid", no?
> 
> Or alternatively we can split the generic part of this driver as soon as a second one comes along and then have both driver include that generic logic.

Yes. I think it is probably the best variant to check if we split this into a front end /back end or provide a new driver when we have something. 
> 
> The only piece where I'm unsure is how this will interact with CRIU. Can containers emulate ioctls and device nodes?
> 
> 
> Alex
> 
> 
> 
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