On 06/14/2010 03:29 PM, Avi Kivity wrote:
On 06/13/2010 03:32 PM, Nadav Har'El wrote:
When KVM wants to inject an interrupt, the guest should think a real
interrupt
has happened. Normally (in the non-nested case) this means checking
that the
guest doesn't block interrupts (and if it does, inject when it
doesn't - using
the "interrupt window" VMX mechanism), and setting up the appropriate
VMCS
fields for the guest to receive the interrupt.
However, when we are running a nested guest (L2) and its hypervisor (L1)
requested exits on interrupts (as most hypervisors do), the most
efficient
thing to do is to exit L2, telling L1 that the exit was caused by an
interrupt, the one we were injecting; Only when L1 asked not to be
notified
of interrupts, we should to inject it directly to the running guest
L2 (i.e.,
the normal code path).
However, properly doing what is described above requires invasive
changes to
the flow of the existing code, which we elected not to do in this stage.
Instead we do something more simplistic and less efficient: we modify
vmx_interrupt_allowed(), which kvm calls to see if it can inject the
interrupt
now, to exit from L2 to L1 before continuing the normal code. The
normal kvm
code then notices that L1 is blocking interrupts, and sets the interrupt
window to inject the interrupt later to L1. Shortly after, L1 gets the
interrupt while it is itself running, not as an exit from L2. The
cost is an
extra L1 exit (the interrupt window).
That's a little sad.
It can also be broken if the guest chooses to keep interrupts disabled
during exits, and instead ask vmx to ack interrupts. The guest can then
vmread the vector number and dispatch the interrupt itself.
--
error compiling committee.c: too many arguments to function
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