* Avi Kivity <[EMAIL PROTECTED]> wrote:
> >NOTE: this is not a worry for upstream kernel, it is caused by
> >PREEMPT_RT scheduling in previously atomic APIs like kunmap(). But
> >KVM used to work pretty nicely in -rt and this problem got introduced
> >fairly recently, related to some big-page changes IIRC.
>
> This is not a recent change. The x86 emulator is fetching an
> instruction or a memory operand.
well, the fact that it's somehow ending up in highmem and thus the kmap
becomes nontrivial could be a recent change maybe?
> > Any suggestions of how to best fix this in -rt? Basically, it would
> > be nice to decouple KVM from get_cpu/preempt_disable type of
> > interfaces as much as possible, to make it fully preemptible.
>
> kvm on intel uses hidden cpu registers, so it can't be migrated as a
> normal process. [...]
oh, granted. My suggestion would be to make it more atomic - not to
break the physical attachment of a KVM context to a physical CPU (which
is unbreakable, obviously).
> [...] We'd need a hook on context switch to check if we migrated to
> another cpu, issue an ipi to unload the registers on the previous cpu,
> and reload the registers on the current cpu. The hook would need to
> be protected from context switches. See vmx.c:vmx_vcpu_load() for the
> gory details. The actual launch would require a small non-preemptible
> section as well, on both intel and AMD.
actually, another, possibly better approach would be to attach a KVM
context to tasks, automatically loaded/unloaded upon switch_to(). No
need to do IPIs. Would this be fast enough?
Ingo
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