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

From: Zachary Amsden
Date: Fri Feb 16 2007 - 05:23:15 EST

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?


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