Re: [PATCH] KVM: VMX cannot yet handle migration-after-preemption, so disable it

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Gregory Haskins wrote:
> On Wed, 2007-07-25 at 06:20 +0300, Avi Kivity wrote:
>
>   
>> I'd rather wait for the real fix, especially as this is an experimental
>> branch.  Can you provide details of the issue?
>>
>>     
>
> Sure.  Keep in mind I am working with the RT kernel, so things might be
> tighter and/or different than the vanilla kernel w.r.t. what I am about
> to say:
>
> But in a nutshell, the current code calls vcpu_clear() during a sched-in
> operation.  If the vcpu is simply uninitialized, no problem.  We clear
> it inline in the handler.  However, if the vcpu is migrating we have an
> issue.  A synchronous smp_call_function_single() is invoked to
> vcpu_clear the remote pCPU, but you must have interrupts enabled to call
> this function to avoid a deadlock in the IPI handshake.  sched-in()
> handlers are (and should be) called with interrupts and preemption
> disabled. 
>
>   

Preemption disabled yes, but interrupts? IIRC I picked the place where
sched_in is called to have interrupts enabled, but maybe -rt changes
that.  Seems like finding a better position for that hook would solve
the problem.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


-------------------------------------------------------------------------
This SF.net email is sponsored by: Splunk Inc.
Still grepping through log files to find problems?  Stop.
Now Search log events and configuration files using AJAX and a browser.
Download your FREE copy of Splunk now >>  http://get.splunk.com/