On Mar 30, 2006, at 8:50 PM, Tian, Kevin wrote:
From: Hollis Blanchard
Sent: 2006年3月31日 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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