Anthony Liguori wrote:
> Howdy,
>
> I had an idea for improving VMEXIT time by only saving/restoring
> sys{enter,call,ret} MSRs when exiting from kernel space code since as
> long as these instructions aren't executed on the host, everything
> should be fine.
>
Yup.
> This worked fine for MSR_IA32_SYSENTER* msrs, but not so much for the
> ?TAR msrs. I have hooks in vcpu_{get,put} but I suspect we're making a
> blocking call somewhere which is resulting in the scheduler being
> invoked (while we still have the vcpu). This then results in generally
> badness in the host.
>
You might compile the host with CONFIG_PREEMPT, which should get you a
nice stacktrace after the "scheduling while atomic" message.
> I'm still a little unclear about this though as I would think that if
> this could happen, it would create problems with VT since KVM another
> guest could run and overwrite the VMCS msr.
>
> I'm I interpreting this correctly? Does disabling preemption not
> guarantee that you won't potentially drop to userspace in your critical
> block?
>
This is only checked under CONFIG_PREEMPT, otherwise preempt_disable
(and get_cpu()) are no-ops.
I really need to enable CONFIG_PREEMPT myself...
--
error compiling committee.c: too many arguments to function
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