Re: [PATCH 1 of 2] libxl: Allow multiple USB devices on HVM domain creation

[George Dunlap <george.dunlap@eu.citrix.com>]

On 29/11/12 11:16, Ian Campbell wrote:
> On Wed, 2012-11-28 at 12:16 +0000, George Dunlap wrote:
>> # HG changeset patch
>> # User George Dunlap <george.dunlap@eu.citrix.com>
>> # Date 1354101445 0
>> # Node ID 538d9ffbd71b41e8cf6d7da0ded9e0a0b07f3c0d
>> # Parent  ae6fb202b233af815466055d9f1a635802a50855
>> libxl: Allow multiple USB devices on HVM domain creation
>>
>> This patch allows an HVM domain to be created with multiple USB
>> devices.
>>
>> Since the previous interface only allowed the passing of a single
>> device, this requires us to add a new element to the hvm struct of
>> libxl_domain_build_info -- usbdevice_list.  For API compatibility, the
>> old element, usbdevice, remains.
>>
>> If b_info->hvm.usbdevice is non-NULL, then it will be used exclusively;
>> otherwise, libxl will check for b_info->hvm.usbdevice_list instead.
> Is this the right way round? If the caller knows about usbdevice_list
> enough to have set it to something then surely that's the one it wanted?

  Well, I had considered returning an error if both were set. :-)  I 
suppose actually that's probably the way that is likely to flag up bugs 
in the toolstack the best -- only doing one or the other will cause 
weird buggy behavior that will be hard to track down.  Does that sound 
good to you?

>> Each device listed will cause an extra "-usbdevice [foo]" to be appended
>> to the qemu command line.
>>
>> In order to allow users of libxl to write software compatible with older
>> versions of libxl, also define LIBXL_HAVE_BUILDINFO_USBDEVICE_LIST.  If
>> this is present, the caller should use b_info->hvm.usbdevice_list; otherwise
>> they should use b_info->hvm.usbdevice.
> Actually it's a both stricter and looser than that, if
> LIBXL_HAVE_BUILDINFO_USBDEVICE_LIST is defined then the caller can
> choose to use usbdevice_list but can also use usbdevice if they wish
> (but not both). If it is not present then they must not use
> usbdevice_list at all.

Hmm, I think I wrote this first and then changed it and forgot to update 
it.  :-)  I also forgot that in this kind of "API spec" document, 
"should" and "must" are technical terms with very specific definitions.  
I'll revise it when I re-spin.

>
> I wonder if this LIBXL_HAVE should also be contained in a #ifdef
> LIBXL_API_VERSION >= 0x040300?

What would be the purpose of that?

  -George