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

Zachary Amsden
2007-02-16T10:22:55Z

Keir Fraser wrote:
> On 16/2/07 07:25, "Jeremy Fitzhardinge" <jeremy@goop.org> 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