Date:   Mon, 27 Feb 2023 13:06:51 -0500
From:   Vivek Goyal <vgoyal@redhat.com>
To:     Stefan Hajnoczi <stefanha@redhat.com>
Cc:     dri-devel@lists.freedesktop.org, helen.koike@collabora.com,
        Al Viro <viro@zeniv.linux.org.uk>,
        Miklos Szeredi <miklos@szeredi.hu>,
        virtualization@lists.linux-foundation.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        wsa+renesas@sang-engineering.com, akpm@linux-foundation.org,
        David Heidelberg <david@ixit.cz>
Subject: Re: [RESEND v2 PATCH] init/do_mounts.c: add virtiofs root fs support
Message-ID: <Y/zxO9PMaES8SenN@redhat.com>
References: <20230224143751.36863-1-david@ixit.cz>
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On Mon, Feb 27, 2023 at 10:53:45AM -0500, Stefan Hajnoczi wrote:
> On Fri, Feb 24, 2023 at 03:37:51PM +0100, David Heidelberg wrote:
> > From: Stefan Hajnoczi <stefanha@redhat.com>
> > 
> > Make it possible to boot directly from a virtiofs file system with tag
> > 'myfs' using the following kernel parameters:
> > 
> >   rootfstype=virtiofs root=myfs rw
> > 
> > Booting directly from virtiofs makes it possible to use a directory on
> > the host as the root file system.  This is convenient for testing and
> > situations where manipulating disk image files is cumbersome.
> > 
> > Reviewed-by: Helen Koike <helen.koike@collabora.com>
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > Signed-off-by: David Heidelberg <david@ixit.cz>
> > ---
> > v2: added Reviewed-by and CCed everyone interested.
> > 
> > We have used this option in Mesa3D CI for testing crosvm for
> > more than one years and it's proven to work reliably.
> > 
> > We are working on effort to removing custom patches to be able to do 
> > automated apply and test of patches from any tree.                              
> > 
> > https://gitlab.freedesktop.org/mesa/mesa/-/blob/main/.gitlab-ci/crosvm-runner.sh#L85
> >  init/do_mounts.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> 
> Vivek, do you remember where we ended up with boot from virtiofs? I
> thought a different solution was merged some time ago.

We merged a patch from Christoph Hellwig to support this.

commit f9259be6a9e7c22d92e5a5000913147ae17e8321
Author: Christoph Hellwig <hch@lst.de>
Date:   Wed Jul 14 16:23:20 2021 -0400

    init: allow mounting arbitrary non-blockdevice filesystems as root

Now one should be able to mount virtiofs using following syntax.

"root=myfs rootfstype=virtiofs rw"

IIUC, this patch should not be required anymore.

Thanks
Vivek

> 
> There is documentation from the virtiofs community here:
> https://virtio-fs.gitlab.io/howto-boot.html
> 
> Stefan
> 
> > 
> > diff --git a/init/do_mounts.c b/init/do_mounts.c
> > index 811e94daf0a8..11c11abe23d7 100644
> > --- a/init/do_mounts.c
> > +++ b/init/do_mounts.c
> > @@ -578,6 +578,16 @@ void __init mount_root(void)
> >  			printk(KERN_ERR "VFS: Unable to mount root fs via SMB.\n");
> >  		return;
> >  	}
> > +#endif
> > +#ifdef CONFIG_VIRTIO_FS
> > +	if (root_fs_names && !strcmp(root_fs_names, "virtiofs")) {
> > +		if (!do_mount_root(root_device_name, "virtiofs",
> > +				   root_mountflags, root_mount_data))
> > +			return;
> > +
> > +		panic("VFS: Unable to mount root fs \"%s\" from virtiofs",
> > +		      root_device_name);
> > +	}
> >  #endif
> >  	if (ROOT_DEV == 0 && root_device_name && root_fs_names) {
> >  		if (mount_nodev_root() == 0)
> > -- 
> > 2.39.1
> >