linux-kernel - Re: [PATCH v4 15/16] virtio-fs: add virtiofs filesystem

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Date:   Wed, 4 Sep 2019 14:58:40 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Stefan Hajnoczi <stefanha@...hat.com>
Cc:     Miklos Szeredi <mszeredi@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Vivek Goyal <vgoyal@...hat.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>
Subject: Re: [PATCH v4 15/16] virtio-fs: add virtiofs filesystem

On Wed, Sep 04, 2019 at 07:16:30PM +0100, Stefan Hajnoczi wrote:
> On Tue, Sep 03, 2019 at 09:55:49AM -0400, Michael S. Tsirkin wrote:
> > On Tue, Sep 03, 2019 at 01:42:02PM +0200, Miklos Szeredi wrote:
> > Endian-ness for fuse header also looks wrong.
> [...]
> > > +struct virtio_fs_forget {
> > > +	struct fuse_in_header ih;
> > > +	struct fuse_forget_in arg;
> > 
> > These structures are all native endian.
> > 
> > Passing them to host will make cross-endian setups painful to support,
> > and hardware implementations impossible.
> > 
> > How about converting everything to LE?
> 
> The driver dictates the endianness of the FUSE protocol session.  The
> virtio-fs device specification states that the device looks at the first
> request's fuse_in_header::opcode field to detect the guest endianness.
> 
> If it sees FUSE_INIT in its native endianness then no byte-swapping is
> necessary.  If it sees FUSE_INIT in the opposite endianness then
> byte-swapping is necessary on the device side.


You are right.  Pls ignore the comment.  We need to reserve the
byte-swapped FUSE_INIT to make sure future versions of fuse don't try to
send that though.  I sent a patch to that effect, let's see whether it
gets accepted.


-- 
MST