Re: [Xen-devel] [PATCH 5 of 9] libxl: only use interactive PyGrub mode when a console is attached

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2011-10-10 at 15:51 +0100, Roger Pau Monnà wrote:
> > That sounds unfortunate.  I agree with you.
> 
> Well Ian Campbell proposed to use a timeout in select and drop data
> when the timeout is hit, that what I've implemented in
> 
> libxl: drop bootloader output if xenconsole file descriptor is not
> available for writing
> 
> The patch is quite simple and solves the problem, I don't think it
> interferes with normal boot process, since a 1s timeout is quite high
> for data to be delivered from xenconsoled_fd to the user if the
> console is attached.

The case where a user types
        xl create /a/domain
        xl console adomain

might have a reasonably long delay in it before the console is
connected. Increasing the libxl buffer size would mitigate that but
you'd still have the problem of not reading from an fd if the cons is !=
0.

Can you print out the actual values of the various producers and
consumers at the point of the hang? (maybe track total bytes too for
debug?)

Perhaps where we have:
        if (xenconsoled_prod == xenconsoled_cons)
            xenconsoled_prod = xenconsoled_cons = 0;
        if (bootloader_prod == bootloader_cons)
            bootloader_prod = bootloader_cons = 0;
We could also add some suitable memmoves and associated prod/cons
manipulation, such that the unconsumed data is always pulled to the head
of the buffer. That might be a reasonably simple way to improve things
and allow larger buffers to be used? More often than not the memmove
won't be getting called since either XXX_prod == XXX_cons or XXX_cons is
already 0, IOW having a partially consumed buffer would be unusual?

Once we have a larger buffer which we always try to fill discarding data
after a timeout when the buffer is full won't be so critical.

>  The other option would be to modify xenconsole
> daemon, but I think that's far too complicated for a problem that can
> be solved easily.
> 
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