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

[Anthony Liguori <anthony@codemonkey.ws>]

Benjamin Herrenschmidt <benh@kernel.crashing.org> writes:

> On Mon, 2013-08-12 at 17:29 +0930, Rusty Russell wrote:
>> virtio data structures are defined as "target endian", which assumes
>> that's a fixed value.  In fact, that actually means it's
>> platform-specific.
>> 
>> Hopefully the OASIS virtio 1.0 spec will fix this.  Meanwhile, create
>> a hook for little endian ppc.
>
> Ok, sorry if I missed a previous debate on that one but why do you do a
> call-out to architecture specific stuff (that is not even inline) on
> every access ?

Let's focus on getting something merged.  Then we can muck around with
it down the road.

Having target-ppc call into virtio is a layering violation.  This
approach keeps the dependencies cleaner.

Regards,

Anthony Liguori

>
> If we consider that virtio byte order is global, can't you make it a
> global that is *set* by the architecture rather than *polled* by
> virtio ?
>
> We have explicit knowledge of when our endianness change (we get the
> hcall or a write to some SPR), we can call virtio *then* to adjust the
> endianness rather than having a call-out to the platform on every
> access.
>
> Ben.