Re: [Qemu-devel] [PATCH 1/8] virtio_get_byteswap: function for endian-ambivalent targets using virtio.

[Cornelia Huck]

On Tue, 18 Feb 2014 17:03:27 +0200
"Michael S. Tsirkin" <address@hidden> wrote:

> On Tue, Feb 18, 2014 at 03:48:38PM +0100, Alexander Graf wrote:
> > On 02/18/2014 01:38 PM, Greg Kurz wrote:
> > >From: Rusty Russell <address@hidden>
> > >
> > >virtio data structures are defined as "target endian", which assumes
> > >that's a fixed value.  In fact, that actually means it's
> > >platform-specific.
> > >
> > >The OASIS virtio 1.0 spec will fix this.  Meanwhile, create a hook for
> > >little endian ppc (and potentially ARM).  This is called at device
> > >reset time (which is done before any driver is loaded) since it
> > >may involve a system call to get the status when running under kvm.
> > >
> > >[ fixed checkpatch.pl error with the virtio_byteswap initialisation,
> > >   ldq_phys() API change, Greg Kurz <address@hidden> ]
> > >Signed-off-by: Rusty Russell <address@hidden>
> > >Signed-off-by: Greg Kurz <address@hidden>
> > >---
> > >  hw/virtio/virtio.c                |    6 ++
> > >  include/hw/virtio/virtio-access.h |  132 
> > > +++++++++++++++++++++++++++++++++++++
> > >  include/hw/virtio/virtio.h        |    2 +
> > >  stubs/Makefile.objs               |    1
> > >  stubs/virtio_get_byteswap.c       |    6 ++
> > >  5 files changed, 147 insertions(+)
> > >  create mode 100644 include/hw/virtio/virtio-access.h
> > >  create mode 100644 stubs/virtio_get_byteswap.c
> > >
> > >diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> > >index aeabf3a..4fd6ac2 100644
> > >--- a/hw/virtio/virtio.c
> > >+++ b/hw/virtio/virtio.c
> > >@@ -19,6 +19,9 @@
> > >  #include "hw/virtio/virtio.h"
> > >  #include "qemu/atomic.h"
> > >  #include "hw/virtio/virtio-bus.h"
> > >+#include "hw/virtio/virtio-access.h"
> > >+
> > >+bool virtio_byteswap;
> > 
> > Could this be a virtio object property rather than a global? Imagine
> > an AMP guest system with a BE and an LE system running in parallel
> > accessing two separate virtio devices. With a single global that
> > would break.
> > 
> > 
> > Alex
> 
> Well, how does a device know which CPU uses it?
> I suspect we are better off waiting for 1.0 with this one.
> 

1.0 makes this a bit more complex, no?

virtio-endian accessors are defined by the endianness of host and guest
(doing a bswap depends on the host/guest combination). This needs to be
per qemu instance. (ioctl under kvm? machine option?)

For 1.0, we'll have everything le, so a be host will always do a bswap
(as will a be guest). But whether a device is 1.0 or legacy is not
something that can be decided globally, or we can't have transitional
devices with qemu.