Re: xl dmesg buffer too small for Xen 4.18?

[Julien Grall <julien@xxxxxxx>]

Hi Roger,

On 19/09/2023 08:02, Roger Pau Monné wrote:
> On Mon, Sep 18, 2023 at 07:49:26PM +0100, Julien Grall wrote:
> > (+Roger and moving to xen-devel)
> >
> > Hi,
> >
> > On 18/09/2023 19:17, Chuck Zmudzinski wrote:
> > > On 9/18/2023 9:00 AM, Chuck Zmudzinski wrote:
> > > > Hello,
> > > >
> > > > I tested Xen 4.18~rc0 on Alma Linux 9 and my first tests indicate it works fine 
> > > > for starting the guests I manage but I notice that immediately after boot and 
> > > > with only dom0 running on the system, I get:
> > > >
> > > > [user@Malmalinux ~]$ sudo xl dmesg
> > > > 00bee72000-00000bee72fff type=7 attr=000000000000000f
> > > > (XEN)  00000bee73000-00000bef49fff type=4 attr=000000000000000f
> > > > (XEN)  00000bef4a000-00000bef4bfff type=7 attr=000000000000000f
> > > > (XEN)  00000bef4c000-00000befbafff type=4 attr=000000000000000f
> > > > (XEN)  00000befbb000-00000befbbfff type=7 attr=000000000000000f
> > > > ...
> > > >
> > > > I have noticed the buffer fills up quickly on earlier Xen versions, but never 
> > > > have I seen it fill up during boot and with only dom0 running.
> > > >
> > > > Can increasing the buffer fix this? How would one do that?
> > > >
> > > > Thanks
> > > I see the setting is the command line option conring_size:
> > >
> > > https://xenbits.xen.org/docs/unstable/misc/xen-command-line.html#conring_size
> > >
> > > The default is 16k, I tried 48k and that was big enough to capture all the 
> > > messages at boot for 4.18 rc0. This is probably not an issue if the release 
> > > candidate is being more verbose than the actual release will be. But if the 
> > > release is still this verbose, maybe the default of 16k should be increased.
> > Thanks for the report. This remind me the series [1] from Roger which tries
> > to increase the default size to 32K. @Roger, I am wondering if we should
> > revive it?
> I think the relevant patch (2/2) will still apply as-is, it's just a
> Kconfig one line change.  I'm however thinking it might be better to
> bump it even further, to 128K.  From a system point of view it's still
> a very small amount of memory.
I don't have a strong opinion about 128K vs 32K.

> I'm happy to repost with an increased buffer size, but only if there's
> someone willing to Ack it, otherwise it's not worth spending time on
> it.
Sorry that patch fell through the cracks. I would be happy to ack the patch.

Cheers,

--
Julien Grall