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

[Alexander Graf]

> Am 18.02.2014 um 16:45 schrieb Cornelia Huck <address@hidden>:
> 
> On Tue, 18 Feb 2014 16:12:08 +0100
> Cornelia Huck <address@hidden> wrote:
> 
>> 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.
> 
> So here are two stupid tables on who needs to do byteswaps, one for
> legacy devices, one for 1.0 devices:
> 
> legacy devices:
> 
>            host
>       be        le
> 
> g be  host no    host yes
> u     guest no   guest no
> e
> s le  host yes   host no
> t     guest no   guest no
> 
> 
> 
> virtio 1.0 devices:
> 
>            host
>       be        le
> 
> g be  host yes   host no
> u     guest yes  guest yes
> e
> s le  host yes   host no
> t     guest no   guest no
> 
> 
> This means byteswaps in qemu always depend on guest-endianness for
> legacy and on host-endianness for 1.0. If we want to support
> transitional devices with a mixture of legacy/1.0, we'll need both a
> per-machine and per-device swap flag:
> 
> virtio_whatever(device, parameters...)
> {
>    if (device->legacy) {
>        if (guest_needs_byteswap) {
>            whatever_byteswap(parameters...);
>        } else {
>            whatever(parameters...);
>        }
>    } else { /* 1.0 */
>        whatever_le(parameters...);
>    }
> }
> 
> Comments?

Yes. My point was that we could move all of the ifery above into the device 
reset function and from then on only use the swap bool property.

Alex

>