Re: [PATCH v2] tools/xl: fix channel configuration setting

["Anthony PERARD" <anthony.perard@xxxxxxxxxx>]

On Thu, Feb 27, 2025 at 04:10:25PM +0100, Jürgen Groß wrote:
> On 26.02.25 16:55, Anthony PERARD wrote:
> > On Tue, Feb 25, 2025 at 08:30:33AM +0100, Juergen Gross wrote:
> > > Channels work differently than other device types: their devid should
> > > be -1 initially in order to distinguish them from the primary console
> > > which has the devid of 0.
> > >
> > > So when parsing the channel configuration, use
> > > ARRAY_EXTEND_INIT_NODEVID() in order to avoid overwriting the devid
> > > set by libxl_device_channel_init().
> > >
> > > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> >
> > This might want a:
> >      Fixes: 3a6679634766 ("libxl: set channel devid when not provided by 
> > application")
> >
> > Before that, the devid set by `xl` was probably ignored. I think before,
> > the console devid would be taken from libxl__init_console_from_channel()
> > parametters, so the first devnum would be `0+1`, so never 0.
> >
> > Do you agree?
>
> I'm not sure I do. The use of ARRAY_EXTEND_INIT() in xl_parse.c predates
> commit 3a6679634766, so there is certainly more than one potential Fixes:
> candidate.

Of course, this was my first candidate as well, but that's part of the
original introduction of channel to the source code, so it didn't really
make sense to that the feature was commited with devid=0, surely it
would already been an issue at the time. This is why I start looking
in libxl on how devid was used.

> So at least for the xl case commit 3a6679634766 isn't relevant, and my patch
> is fixing the xl case only.

Last time I check, `xl` is using libxl, so any change to libxl can
impact xl.

Before 3a6679634766, channel#0 would be connected to console#1 because
libxl__init_console_from_channel() was called with dev_num=`channel#+1`,
and $dev_num was used to set console.devid.

After 3a6679634766, we have console.devid=channel.devid=dev_num if
channel.devid is -1, or console.devid=channel.devid.

Without 3a6679634766, having more than one channel with devid=-1 would
result in failing to start due to having more than one channel with the
same devid.

So the only good candidate is:
Fixes: 3a6679634766 ("libxl: set channel devid when not provided by 
application")


Also, I'm not sure channels are going to work properly if there's more
than one console, lots of code seems to assume
console.devid=channel.devid+1, and there's probably other issues.

Cheers,

--

Anthony Perard | Vates XCP-ng Developer

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