Re: [Qemu-devel] [PATCH v8 00/20] virtio endian-ambivalent target

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, Jun 18, 2014 at 02:35:21PM +0200, Greg Kurz wrote:
> On Wed, 18 Jun 2014 18:38:14 +0800
> Stefan Hajnoczi <stefanha@gmail.com> wrote:
> 
> > On Tue, Jun 17, 2014 at 09:40:19AM +0200, Alexander Graf wrote:
> > > 
> > > On 17.06.14 09:36, Stefan Hajnoczi wrote:
> > > >On Fri, Jun 13, 2014 at 01:18:00PM +0200, Greg Kurz wrote:
> > > >>This version merges the changes requested during the v7 review, remarks from
> > > >>ppc64 dump support review (yes, we talked about virtio there) and the work on
> > > >>virtio subsections migration. Also two new patches have been added:
> > > >>- patch #1 is a preliminary fix for virtio-serial posted by Alexander Graf
> > > >>- patch #9 prepares the work on the virtio_is_big_endian() helper
> > > >>
> > > >>The most significant changes are:
> > > >>- introduction of a new CPU method for virtio
> > > >>- endianness is taken from CPU that resets the device
> > > >>- fastpath virtio memory accessors for fixed endian targets
> > > >>- VMState based virtio subsections (compatibility friendly)
> > > >I'm surprised it's not enough for the virtio device to have an
> > > >endianness field (big/little).  It seems these patches make endianness
> > > >depend on the CPUState through which the device is being accessed.
> > > >
> > > >Can you explain why it's necessary to check the CPUState?
> > > 
> > > They only check CPUState at the point in time of reset, as that's the only
> > > case where we can derive the implicit endian configuration from :).
> > 
> > What bothers me is that real hardware can't do this.  Given that VIRTIO
> > 1.0 is always little-endian I guess this is just a temporary hack for
> > ppc little-endian.  Would be nice to add a comment so it's clear why
> > this approach is being taken instead of a cleaner solution.
> > 
> > Stefan
> 
> Hi Stefan,
> 
> Previous versions of this patch set had such comments:
> 
> "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, by making all little endian.
> 
> We need to support both implementations and we want to share as much code
> as possible."
> 
> but these lines got lost between v6 and v7... my bad... :-\

Thanks!  I just wasn't mentally prepared when looking through the patch
series, the comment puts it into context.

Stefan

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