Re: [Xen-devel] Re: [patch 14/21] Xen-paravirt: Add XEN config options and disable unsupported config options.

[Zachary Amsden <zach@xxxxxxxxxx>]

Keir Fraser wrote:
> On 16/2/07 07:25, "Jeremy Fitzhardinge" <jeremy@xxxxxxxx> wrote:
>
>   
> > > Oh, so that's why it doesn't break when CONFIG_PREEMPT=y.  In which case
> > > that preempt_disable() I spotted is wrong-and-unneeded.
> > >
> > > Why doesn't Xen work with preemption??
> > >       
> > I've forgotten the details.  Ian?  Keir?  Steven?  Maybe it can be done.
> >     
> It breaks guest save/restore for us currently because threads can be
> sleeping with machine addresses in local storage (registers, stack). There
> are a few ways to achieve an acceptable solution:
>
>  1. Put processes in the freezer when we suspend. This should avoid any
> thread being in a critical section with machine addresses in its hand. We
> haven't yet investigated the performance impact of freezing processes,
> particularly on the downtime of live relocation.
>
>  2. Allow CONFIG_PREEMPT to be compiled in, but disable it at runtime. We
> could do this by, for example, reserving a bit in preempt_count() so that
> most preemption checks do not touch any more cache lines. I guess it would
> need a bit of fixing up (e.g., so that in_atomic() would not be always
> asserted). Even better for us would be to allow switching between
> involuntary and voluntary preemption at runtime. It looks as though the hook
> points for these two techniques are not usually compiled in at the same
> time, however.
>
>   
Doesn't stop_machine_run already take care of getting you out of all 
kernel threads?  So you can only be sleeping, not preempted, in which 
case, this might not be an issue?
Zach


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