[RFC] Handling VMEXITS with interrupts disabled?

[Anthony Liguori <anthony-rdkfGonbjUSkNkDKm+mE6A@public.gmane.org>]

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.

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.

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?

Regards,

Anthony Liguori

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