Keir Fraser wrote: > On 16/2/07 07:25, "Jeremy Fitzhardinge" <jeremy at 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
