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

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Message-ID: <20190904181630.GB26826@stefanha-x1.localdomain>
Date:   Wed, 4 Sep 2019 19:16:30 +0100
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     "Michael S. Tsirkin" <mst@...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 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.

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