linux-kernel - Re: [PATCH v8] drivers/misc: sysgenid: add system generation id driver

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Date:   Mon, 8 Mar 2021 17:03:58 +0100
From:   Alexander Graf <graf@...zon.com>
To:     Greg KH <gregkh@...uxfoundation.org>,
        Adrian Catangiu <acatan@...zon.com>
CC:     <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <qemu-devel@...gnu.org>, <kvm@...r.kernel.org>,
        <linux-s390@...r.kernel.org>, <rdunlap@...radead.org>,
        <arnd@...db.de>, <ebiederm@...ssion.com>, <rppt@...nel.org>,
        <0x7f454c46@...il.com>, <borntraeger@...ibm.com>,
        <Jason@...c4.com>, <jannh@...gle.com>, <w@....eu>,
        <colmmacc@...zon.com>, <luto@...nel.org>, <tytso@....edu>,
        <ebiggers@...nel.org>, <dwmw@...zon.co.uk>, <bonzini@....org>,
        <sblbir@...zon.com>, <raduweis@...zon.com>, <corbet@....net>,
        <mst@...hat.com>, <mhocko@...nel.org>, <rafael@...nel.org>,
        <pavel@....cz>, <mpe@...erman.id.au>, <areber@...hat.com>,
        <ovzxemul@...il.com>, <avagin@...il.com>,
        <ptikhomirov@...tuozzo.com>, <gil@...l.com>, <asmehra@...hat.com>,
        <dgunigun@...hat.com>, <vijaysun@...ibm.com>, <oridgar@...il.com>,
        <ghammer@...hat.com>
Subject: Re: [PATCH v8] drivers/misc: sysgenid: add system generation id
 driver



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.


Alex



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