Re: [Xen-devel] [patch] make hypercall_preempt_check() a little moresensitive

[Jimi Xenidis <jimix@xxxxxxxxxxxxxx>]

On Mar 30, 2006, at 8:50 PM, Tian, Kevin wrote:

> > From: Hollis Blanchard
> > Sent: 2006å3æ31æ 1:17
> >
> > Since PowerPC has a hypervisor mode in the processor, Linux is  
> > able to
> > disable
> > interrupts directly using the EE bit in the MSR (Machine State  
> > Register),
> > just like on hardware. This bit acts like the evtchn_upcall_mask bit.
> A bit curious. Do you mean xenlinux can operate real MSR directly,
Yes, some bits can be controlled by xenlinux

> to
> disable real external interrupts on that physical processor,
Yes, even this bit, tho' it this is on the 970 class of processor,  
newer processors can be configured to not disable externals, but they  
can still set the bit.
> or here
> so-called MSR is a virtual MSR presented per guest to disable virtual
> interrupt on that virtual processor? I prefer to the latter. :-)
I understand your preference, but we've found with other hypervisors,  
that under server class workloads it does not matter.  I could see  
how this would be a big deal for real-time, but Xen doesn't do that..  
yet :)
> By following your approach, there're others places you may need  
> similar
> wrapper to make it robust. For example, unmask_evtchn in
> xenlinux/drivers/xen/core/evtchn.c will check evtchn_upcall_mask to  
> see
> whether force callback to fire previous pending events before  
> unmasking.
> Based on your description for powerpc, there you also need to check  
> the
> EE flag to avoid unnecessary force back since interrupt may be  
> disabled
> that time.
This is an interesting example,  but why deliver an upcall thru the  
HV in the first place, why not just call the evtchn handler directly?
> So hypercall_preempt_check is only one place directly related to  
> that bug.
hmm, this is quite the pathological case, but I cannot deny it will  
hit us else where.
We expect to explore the Linux change when the xen-patches make it to  
kernel.org.
-JX
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