Re: [Xen-devel] Fix VGA logdirty related display freezes with altp2m

[Tamas K Lengyel <tamas.k.lengyel@xxxxxxxxx>]

On Wed, Oct 24, 2018 at 11:31 AM Tamas K Lengyel
<tamas.k.lengyel@xxxxxxxxx> wrote:
>
> On Wed, Oct 24, 2018 at 11:20 AM Razvan Cojocaru
> <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> >
> > On 10/24/18 8:09 PM, Tamas K Lengyel wrote:
> > > On Tue, Oct 23, 2018 at 6:37 AM Razvan Cojocaru
> > > <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> > >>
> > >> Tamas, could you please give this a spin?
> > >>
> > >> https://github.com/razvan-cojocaru/xen/tree/altp2m-logdirty-take2
> > >>
> > >> It _should_ solve the crashes.
> > >
> > > Indeed, I no longer see the crash. However, there might be some
> > > locking issue present because the whole system freezes up shortly
> > > after starting DRAKVUF on a domain - within a couple seconds. I mean
> > > Xen itself locks up: no response on the serial, dom0 screen frozen,
> > > etc.
> >
> > Do you have any type of log / backtrace / way I could reproduce it
> > without Drakvuf? All the ways I've tested it were fine (including
> > xen-access).
>
> I don't have a standalone test that produces that error. With DRAKVUF
> it is easily reproducible though. If you have a Windows guest
> installed, setting up DRAKVUF should really not be much trouble. With
> xen-access it indeed doesn't lock up but since the guest is pretty
> much unresponsive during that test I can't verify whether the VGA
> issue is now resolved or not. Also the xen-access tests are fairly
> limited and don't use all aspects of altp2m.
>

What I see from the DRAKVUF log is that the last thing it prints is
sending a vm_event response that both enables singlestepping and
switches altp2m view. This looks to be consistent. It didn't matter if
the guest had 1 or 2 vCPUs, the freeze occurs just the same. It's
definitely racey because it doesn't happen right away, the system
works as expected for a couple seconds.

Tamas

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