Re: [Xen-devel] [PATCH v4 1/8] xen: import ring.h from xen

[Juergen Gross <jgross@xxxxxxxx>]

On 23/03/17 19:22, Stefano Stabellini wrote:
> On Thu, 23 Mar 2017, Paolo Bonzini wrote:
>> On 23/03/2017 14:55, Juergen Gross wrote:
>>> On 23/03/17 14:00, Greg Kurz wrote:
>>>> On Mon, 20 Mar 2017 11:19:05 -0700
>>>> Stefano Stabellini <sstabellini@xxxxxxxxxx> wrote:
>>>>
>>>>> Do not use the ring.h header installed on the system. Instead, import
>>>>> the header into the QEMU codebase. This avoids problems when QEMU is
>>>>> built against a Xen version too old to provide all the ring macros.
>>>>>
>>>>> Signed-off-by: Stefano Stabellini <stefano@xxxxxxxxxxx>
>>>>> Reviewed-by: Greg Kurz <groug@xxxxxxxx>
>>>>> CC: anthony.perard@xxxxxxxxxx
>>>>> CC: jgross@xxxxxxxx
>>>>> ---
>>>>> NB: The new macros have not been committed to Xen yet. Do not apply this
>>>>> patch until they do.
>>>>> ---
>>>>
>>>> Looking at your other series for the kernel part of this feature:
>>>>
>>>> https://lkml.org/lkml/2017/3/22/761
>>>>
>>>> I realize that the ring.h header from Xen also exists in the kernel 
>>>> tree... 
>>>>
>>>> Shouldn't all the code that can be used in both kernel and userspace go to 
>>>> a
>>>> header file under include/uapi in the kernel tree ? And then we would 
>>>> import
>>>> it under include/standard-headers/linux in the QEMU tree and we could keep 
>>>> it
>>>> in sync using scripts/update-linux-headers.sh.
>>>>
>>>> Cc'ing Paolo for insights.
>>>
>>> As Xen isn't part of the kernel we don't want that. You can use and/or
>>> build qemu with xen-9pfs backend support on an old Linux kernel without
>>> the related frontend.
>>
>> As long as the header changes rarely, I guess it's fine not to go
>> through update-linux-headers.sh.
> 
> Very rarely, last time ring.h was changed was 2015, and to introduce a
> new macro (which we don't necessarily need in QEMU).
> 
> 
>>> OTOH I don't see the advantage of not using the headers from Xen. This
>>> is working for qdisk and pvusb backends and for all the Xen libraries.
>>> Do you expect the 9pfs backend to be used for a qemu version built
>>> against a Xen version not supporting that backend?
> 
> Yes, I think that is entirely possible: Xen and QEMU versions can mix
> and match.
> 
> Keeping in mind that the 9pfs backend has actually no build dependencies
> on Xen, except for these new ring.h macros, we have the following
> options:
> 
> 1) we build the 9pfs backend only for Xen >= 4.9, because of the new
>    macros in ring.h that we need

Right. You have sent 9pfs support patches for Xen tools. So obviously
you need a proper Xen version to use 9pfs. Why not build qemu against
it? Do you really expect a new Xen being used with an old qemu while
wanting to use new features? That makes no sense for me.


Juergen

PS: added xen-devel as this should be discussed there, too.

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