linux-kernel - Re: [PATCH 2/2] virt: vmgenid: add support for generation counter

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Date:   Wed, 3 Aug 2022 17:28:08 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     bchalios@...zon.es
Cc:     linux-kernel@...r.kernel.org, tytso@....edu, Jason@...c4.com,
        dwmw@...zon.co.uk, graf@...zon.de, xmarcalx@...zon.co.uk
Subject: Re: [PATCH 2/2] virt: vmgenid: add support for generation counter

On Wed, Aug 03, 2022 at 05:21:27PM +0200, bchalios@...zon.es wrote:
> From: Babis Chalios <bchalios@...zon.es>
> 
> VM Generation ID provides a means of reseeding kernel's RNG using a
> 128-bit UUID when a VM fork occurs, thus avoiding issues running
> multiple VMs with the exact same RNG state. However, user-space
> applications, such as user-space PRNGs and applications that maintain
> world-unique data, need a mechanism to handle VM fork events as well.
> 
> To handle the user-space use-case, this: <url> qemu patch extends
> Microsoft's original vmgenid specification adding an extra page which
> holds a single 32-bit generation counter, which increases every time a
> VM gets restored from a snapshot.
> 
> This patch exposes the generation counter through a character device
> (`/dev/vmgenid`) that provides a `read` and `mmap` interface, for
> user-space applications to consume. Userspace applications should read
> this value before starting a transaction involving cached random bits
> and ensure that it has not changed while committing the transaction.
> 
> It can be used from qemu using the `-device vmgenid,guid=auto,genctr=42`
> parameter to start a VM with a generation counter with value 42.
> Reading 4 bytes from `/dev/vmgenid` will return the value 42. Next, use
> `savevm my_snapshot` in the monitor to snapshot the VM. Now, start
> another VM using `-device vmgenid,guid=auto,genctr=43 -loadvm
> my_snapshot`. Reading now from `/dev/vmgenid` will return 43.
> 
> Signed-off-by: Babis Chalios <bchalios@...zon.es>
> ---
>  Documentation/virt/vmgenid.rst | 120 +++++++++++++++++++++++++++++++++
>  drivers/virt/vmgenid.c         | 103 +++++++++++++++++++++++++++-
>  2 files changed, 221 insertions(+), 2 deletions(-)
>  create mode 100644 Documentation/virt/vmgenid.rst
> 
> diff --git a/Documentation/virt/vmgenid.rst b/Documentation/virt/vmgenid.rst
> new file mode 100644
> index 000000000..61c29e4a7
> --- /dev/null
> +++ b/Documentation/virt/vmgenid.rst
> @@ -0,0 +1,120 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +=======
> +VMGENID
> +=======

<snip>

This file is now just floating in the directory, not tied to anything,
so auto-generation of the documentation will not pick it up or link to
it, right?

So, why does this have to be a separate file at all?  Why not put this
in the .c file and pull it straight from there so that it keeps in sync
with the code easier?

thanks,

greg k-h