Re: [Xen-devel] [PATCH 00/25 v7] SBSA UART emulation support in Xen

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Fri, Aug 18, 2017 at 02:30:03PM +0100, Julien Grall wrote:
> Hi Wei,
> 
> On 15/08/17 10:49, Wei Liu wrote:
> > On Thu, Aug 10, 2017 at 06:51:24PM +0100, Julien Grall wrote:
> > > > > > The interface between Xen and xenconsoled can be asynchronous, it 
> > > > > > can
> > > > > > opt to queue X characters before sending an event, also setup a 
> > > > > > oneshot
> > > > > > timer to avoid hanging.
> > > > > > 
> > > > > > This however has some other implications -- it might not be as 
> > > > > > reliable
> > > > > > as the original method because data is not guaranteed to hit 
> > > > > > backend. If
> > > > > > the guest crashes very early on, depending the actual 
> > > > > > implementation you
> > > > > > might not be able get the data.
> > > > > 
> > > > > Would it be possible to ask xenconsoled to dump everything on domain 
> > > > > crash?
> > > > > Some kind of synchronization.
> > > > > 
> > > > 
> > > > No, not at the moment. If the data is still in Xen and destroyed,
> > > > xenconsoled can't do anything.
> > > 
> > > The vUART emulation is directly queuing the data, there are no 
> > > intermediate
> > > buffer. So all the data would be in the shared ring available for
> > > xenconsoled to go through.
> > > 
> > > It would be quite a useful enhancement for when the guest crash.
> > > 
> > 
> > What I meant was actually "If the data is still in the ring". There is
> > no synchronisation between Xen and xenconsoled to let the latter pull
> > out all data before destroying the guest.
> 
> I don't think you would need synchronisation between Xen and xenconsoled at
> domain destruction. Given that xenconsoled would have to unmap the page, I
> was suggesting that it makes sure that cons == prod before destroying the
> instance. So all the character queued would be displayed on the console.

So this is relying on Dom0 holding a reference to the page so that the
page won't be freed. This sounds plausible.

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