linux-kernel - Re: [PATCH] vhost/vsock: specify module version

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Message-ID: <CAEivzxdiEu3Tzg7rK=TqDg4Ats-H+=JiPjvZRAnmqO7-jZv2Zw@mail.gmail.com>
Date: Sun, 29 Sep 2024 19:35:35 +0200
From: Aleksandr Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: stefanha@...hat.com, Stefano Garzarella <sgarzare@...hat.com>, 
	Jason Wang <jasowang@...hat.com>, Eugenio Pérez <eperezma@...hat.com>, 
	kvm@...r.kernel.org, virtualization@...ts.linux.dev, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] vhost/vsock: specify module version

On Sun, Sep 29, 2024 at 6:56 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Thu, Sep 26, 2024 at 06:16:40PM +0200, Alexander Mikhalitsyn wrote:
> > Add an explicit MODULE_VERSION("0.0.1") specification
> > for a vhost_vsock module. It is useful because it allows
> > userspace to check if vhost_vsock is there when it is
> > configured as a built-in.
> >
> > Without this change, there is no /sys/module/vhost_vsock directory.
> >
> > With this change:
> > $ ls -la /sys/module/vhost_vsock/
> > total 0
> > drwxr-xr-x   2 root root    0 Sep 26 15:59 .
> > drwxr-xr-x 100 root root    0 Sep 26 15:59 ..
> > --w-------   1 root root 4096 Sep 26 15:59 uevent
> > -r--r--r--   1 root root 4096 Sep 26 15:59 version
> >
> > Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
>
>

Dear Michael,

> Why not check that the misc device is registered?

It is possible to read /proc/misc and check if "241 vhost-vsock" is
there, but it means that userspace
needs to have a specific logic for vsock. At the same time, it's quite
convenient to do something like:
    if [ ! -d /sys/modules/vhost_vsock ]; then
        modprobe vhost_vsock
    fi

> I'd rather not add a new UAPI until actually necessary.

I don't insist. I decided to send this patch because, while I was
debugging a non-related kernel issue
on my local dev environment I accidentally discovered that LXD
(containers and VM manager)
fails to run VMs because it fails to load the vhost_vsock module (but
it was built-in in my debug kernel
and the module file didn't exist). Then I discovered that before
trying to load a module we
check if /sys/module/<module name> exists. And found that, for some
reason /sys/module/vhost_vsock
does not exist when vhost_vsock is configured as a built-in, and
/sys/module/vhost_vsock *does* exist when
vhost_vsock is loaded as a module. It looks like an inconsistency and
I also checked that other modules in
drivers/vhost have MODULE_VERSION specified and version is 0.0.1. I
thought that this change looks legitimate
and convenient for userspace consumers.

Kind regards,
Alex

>
> > ---
> >  drivers/vhost/vsock.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
> > index 802153e23073..287ea8e480b5 100644
> > --- a/drivers/vhost/vsock.c
> > +++ b/drivers/vhost/vsock.c
> > @@ -956,6 +956,7 @@ static void __exit vhost_vsock_exit(void)
> >
> >  module_init(vhost_vsock_init);
> >  module_exit(vhost_vsock_exit);
> > +MODULE_VERSION("0.0.1");
> >  MODULE_LICENSE("GPL v2");
> >  MODULE_AUTHOR("Asias He");
> >  MODULE_DESCRIPTION("vhost transport for vsock ");
> > --
> > 2.34.1
>