Re: [PATCH 0/3] virtio-mmio: handle BE guests on LE hosts

Michael S. Tsirkin
2013-10-15T06:38:31Z

On Tue, Oct 15, 2013 at 09:53:17AM +1030, Rusty Russell wrote:
> "Michael S. Tsirkin" <mst@redhat.com> writes:
> > On Mon, Oct 14, 2013 at 06:50:48PM +0200, Paolo Bonzini wrote:
> >> Il 14/10/2013 17:36, Marc Zyngier ha scritto:
> >> >> >
> >> >> > Devices are fine in QEMU, it's only the "generic" parts (rings) that are
> >> >> > missing AFAICT.
> >> > So if I understand correctly how it works, target endianness is set at
> >> > compile time, and you have a BE specific QEMU?
> >>
> >> Yes. Though as Alex said, this will have to change for PPC
> >> little-endian support.
> >>
> >> Paolo
> >
> > Or we'll just say that this platform requires virtio 1.0.
>
> To come full circle, I have implemented virtio support for BE PPC in
> qemu :) And we'll be supporting pre-1.0 for the moment. I'm awaiting
> some ppc-specific hooks, but it could be merged without them.
>
> It's icky: we don't really want to use the current endianness of the
> CPU, since in theory a BE kernel could be running an LE process. So at
> the time of virtio device reset (the first thing the Linux drivers do)
> we read the endianness of the interrupt entry point, which is presumably
> the OS endianness.

Indeed, ick.
The next thing after reset is status write:

/* We always start by resetting the device, in case a previous
* driver messed it up. This also tests that code path a
* little. */
dev->config->reset(dev);

/* Acknowledge that we've seen the device. */
add_status(dev, VIRTIO_CONFIG_S_ACKNOWLEDGE);

and mmio happens to use a 32 bit register for this:

static void vm_set_status(struct virtio_device *vdev, u8 status)
{
struct virtio_mmio_device *vm_dev = to_virtio_mmio_device(vdev);

/* We should never be setting status to 0. */
BUG_ON(status == 0);

writel(status, vm_dev->base + VIRTIO_MMIO_STATUS);
}

so if we only care about mmio, we can use the status write
to detect endian-ness: low byte set means LE, high byte set
means BE, which seems cleaner.

Hmm?

--
MST