• Sean Christopherson's avatar
    KVM: VMX: use preemption timer to force immediate VMExit · d264ee0c
    Sean Christopherson authored
    A VMX preemption timer value of '0' is guaranteed to cause a VMExit
    prior to the CPU executing any instructions in the guest.  Use the
    preemption timer (if it's supported) to trigger immediate VMExit
    in place of the current method of sending a self-IPI.  This ensures
    that pending VMExit injection to L1 occurs prior to executing any
    instructions in the guest (regardless of nesting level).
    
    When deferring VMExit injection, KVM generates an immediate VMExit
    from the (possibly nested) guest by sending itself an IPI.  Because
    hardware interrupts are blocked prior to VMEnter and are unblocked
    (in hardware) after VMEnter, this results in taking a VMExit(INTR)
    before any guest instruction is executed.  But, as this approach
    relies on the IPI being received before VMEnter executes, it only
    works as intended when KVM is running as L0.  Because there are no
    architectural guarantees regarding when IPIs are delivered, when
    running nested the INTR may "arrive" long after L2 is running e.g.
    L0 KVM doesn't force an immediate switch to L1 to deliver an INTR.
    
    For the most part, this unintended delay is not an issue since the
    events being injected to L1 also do not have architectural guarantees
    regarding their timing.  The notable exception is the VMX preemption
    timer[1], which is architecturally guaranteed to cause a VMExit prior
    to executing any instructions in the guest if the timer value is '0'
    at VMEnter.  Specifically, the delay in injecting the VMExit causes
    the preemption timer KVM unit test to fail when run in a nested guest.
    
    Note: this approach is viable even on CPUs with a broken preemption
    timer, as broken in this context only means the timer counts at the
    wrong rate.  There are no known errata affecting timer value of '0'.
    
    [1] I/O SMIs also have guarantees on when they arrive, but I have
        no idea if/how those are emulated in KVM.
    Signed-off-by: default avatarSean Christopherson <sean.j.christopherson@intel.com>
    [Use a hook for SVM instead of leaving the default in x86.c - Paolo]
    Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
    d264ee0c
x86.c 246 KB