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Date:   Mon, 27 Apr 2020 05:20:17 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Richard Earnshaw <Richard.Earnshaw@....com>,
        Sudeep Dutt <sudeep.dutt@...el.com>,
        Ashutosh Dixit <ashutosh.dixit@...el.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jason Wang <jasowang@...hat.com>,
        netdev <netdev@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        KVM list <kvm@...r.kernel.org>,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH v4] vhost: disable for OABI

On Mon, Apr 27, 2020 at 08:45:22AM +0200, Geert Uytterhoeven wrote:
> Hi Michael,
> 
> Thanks for your patch!
> 
> On Mon, Apr 20, 2020 at 5:13 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> > vhost is currently broken on the some ARM configs.
> >
> > The reason is that the ring element addresses are passed between
> > components with different alignments assumptions. Thus, if
> > guest selects a pointer and host then gets and dereferences
> > it, then alignment assumed by the host's compiler might be
> > greater than the actual alignment of the pointer.
> > compiler on the host from assuming pointer is aligned.
> >
> > This actually triggers on ARM with -mabi=apcs-gnu - which is a
> > deprecated configuration. With this OABI, compiler assumes that
> > all structures are 4 byte aligned - which is stronger than
> > virtio guarantees for available and used rings, which are
> > merely 2 bytes. Thus a guest without -mabi=apcs-gnu running
> > on top of host with -mabi=apcs-gnu will be broken.
> >
> > The correct fix is to force alignment of structures - however
> > that is an intrusive fix that's best deferred until the next release.
> >
> > We didn't previously support such ancient systems at all - this surfaced
> > after vdpa support prompted removing dependency of vhost on
> > VIRTULIZATION. So for now, let's just add something along the lines of
> >
> >         depends on !ARM || AEABI
> >
> > to the virtio Kconfig declaration, and add a comment that it has to do
> > with struct member alignment.
> >
> > Note: we can't make VHOST and VHOST_RING themselves have
> > a dependency since these are selected. Add a new symbol for that.
> 
> Adding the dependencies to VHOST and VHOST_RING themselves is indeed not
> sufficient.  But IMHO you should still add VHOST_DPN dependencies t
>  these two symbols, so any driver selecting them without fulfilling the
> VHOST_DPN dependency will trigger a Kconfig warning.  Else the
> issue will be ignored silently.

Good point.
For now I'm trying to just get rid of this work around.
If I can't I will add the suggested change.
Thanks!

> > We should be able to drop this dependency down the road.
> >
> > Fixes: 20c384f1ea1a0bc7 ("vhost: refine vhost and vringh kconfig")
> > Suggested-by: Ard Biesheuvel <ardb@...nel.org>
> > Suggested-by: Richard Earnshaw <Richard.Earnshaw@....com>
> > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> 
> Gr{oetje,eeting}s,
> 
>                         Geert
> 
> -- 
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org
> 
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                 -- Linus Torvalds

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