Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Mon, 8 Mar 2021 18:24:44 +0100
From:   Greg KH <gregkh@linuxfoundation.org>
To:     Alexander Graf <graf@amazon.com>
Cc:     Adrian Catangiu <acatan@amazon.com>, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, qemu-devel@nongnu.org,
        kvm@vger.kernel.org, linux-s390@vger.kernel.org,
        rdunlap@infradead.org, arnd@arndb.de, ebiederm@xmission.com,
        rppt@kernel.org, 0x7f454c46@gmail.com, borntraeger@de.ibm.com,
        Jason@zx2c4.com, jannh@google.com, w@1wt.eu, colmmacc@amazon.com,
        luto@kernel.org, tytso@mit.edu, ebiggers@kernel.org,
        dwmw@amazon.co.uk, bonzini@gnu.org, sblbir@amazon.com,
        raduweis@amazon.com, corbet@lwn.net, mst@redhat.com,
        mhocko@kernel.org, rafael@kernel.org, pavel@ucw.cz,
        mpe@ellerman.id.au, areber@redhat.com, ovzxemul@gmail.com,
        avagin@gmail.com, ptikhomirov@virtuozzo.com, gil@azul.com,
        asmehra@redhat.com, dgunigun@redhat.com, vijaysun@ca.ibm.com,
        oridgar@gmail.com, ghammer@redhat.com
Subject: Re: [PATCH v8] drivers/misc: sysgenid: add system generation id
 driver
Message-ID: <YEZd3It1llq9ctPN@kroah.com>
References: <1615213083-29869-1-git-send-email-acatan@amazon.com>
 <YEY2b1QU5RxozL0r@kroah.com>
 <a61c976f-b362-bb60-50a5-04073360e702@amazon.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <a61c976f-b362-bb60-50a5-04073360e702@amazon.com>
Precedence: bulk

On Mon, Mar 08, 2021 at 05:03:58PM +0100, Alexander Graf wrote:
> 
> 
> On 08.03.21 15:36, Greg KH wrote:
> > 
> > On Mon, Mar 08, 2021 at 04:18:03PM +0200, Adrian Catangiu wrote:
> > > +static struct miscdevice sysgenid_misc = {
> > > +     .minor = MISC_DYNAMIC_MINOR,
> > > +     .name = "sysgenid",
> > > +     .fops = &fops,
> > > +};
> > 
> > Much cleaner, but:
> > 
> > > +static int __init sysgenid_init(void)
> > > +{
> > > +     int ret;
> > > +
> > > +     sysgenid_data.map_buf = get_zeroed_page(GFP_KERNEL);
> > > +     if (!sysgenid_data.map_buf)
> > > +             return -ENOMEM;
> > > +
> > > +     atomic_set(&sysgenid_data.generation_counter, 0);
> > > +     atomic_set(&sysgenid_data.outdated_watchers, 0);
> > > +     init_waitqueue_head(&sysgenid_data.read_waitq);
> > > +     init_waitqueue_head(&sysgenid_data.outdated_waitq);
> > > +     spin_lock_init(&sysgenid_data.lock);
> > > +
> > > +     ret = misc_register(&sysgenid_misc);
> > > +     if (ret < 0) {
> > > +             pr_err("misc_register() failed for sysgenid\n");
> > > +             goto err;
> > > +     }
> > > +
> > > +     return 0;
> > > +
> > > +err:
> > > +     free_pages(sysgenid_data.map_buf, 0);
> > > +     sysgenid_data.map_buf = 0;
> > > +
> > > +     return ret;
> > > +}
> > > +
> > > +static void __exit sysgenid_exit(void)
> > > +{
> > > +     misc_deregister(&sysgenid_misc);
> > > +     free_pages(sysgenid_data.map_buf, 0);
> > > +     sysgenid_data.map_buf = 0;
> > > +}
> > > +
> > > +module_init(sysgenid_init);
> > > +module_exit(sysgenid_exit);
> > 
> > So you do this for any bit of hardware that happens to be out there?
> > Will that really work?  You do not have any hwid to trigger off of to
> > know that this is a valid device you can handle?
> 
> The interface is already useful in a pure container context where the
> generation change request is triggered by software.
> 
> And yes, there are hardware triggers, but Michael was quite unhappy about
> potential races between VMGenID change and SysGenID change and thus wanted
> to ideally separate the interfaces. So we went ahead and isolated the
> SysGenID one, as it's already useful as is.
> 
> Hardware drivers to inject change events into SysGenID can then follow
> later, for all different hardware platforms. But SysGenID as in this patch
> is a completely hardware agnostic concept.

Ok, so what is going to cause this driver to be automatically loaded?
How will userspace "know" they want this and know to load it?

This really is just a shared memory "driver", it's gotten so small now,
so why can't this just be a userspace program/server now?  :)

thanks,

greg k-h