Re: [PATCH]: pointer to vmcs getting lost

[Marcelo Tosatti <mtosatti@redhat.com>]

Hi Jesse,

On Fri, Aug 01, 2008 at 03:18:52PM -0700, Jesse wrote:
> Greetings,
>
> I noticed a race condition when running two guests simultaneously and  
> debugging both guests (on 64-bit intel cpus). Periodically I would get  
> errors from the vmread, vmwrite, or vmresume instructions. Some research  
> revealed that these errors were being caused by having an invalid vmcs  
> loaded. Further, I found that the vmcs is a per_cpu variable, which I  
> believe means that any reference to it is invalid after a context  
> switch. (Corrections appreciated). This means that the vmcs must be  
> reloaded each time the process is switched to. 

The preempt notifiers will do that for you.

> The patch below fixed the  
> problem for me.
>
> This patch does three things.
> 1. Extends the critical section in __vcpu_run to include the handling of  
> vmexits, where many of the vmread/writes occur.
> 2. Perform a vcpu_load after we enter the critical section, and after we  
> return from kvm_resched.
> 3. Move the call to kvm_guest_debug_pre into the critical section  
> (because it calls vmread/write).

Wouldnt it suffice to move ->guest_debug_pre into the non preemptable
section? http://article.gmane.org/gmane.comp.emulators.kvm.devel/20244 

I haven't tested that patch though.