• Radim Krčmář's avatar
    KVM: i8254: change PIT discard tick policy · 5fd34264
    Radim Krčmář authored
    commit 7dd0fdff upstream.
    
    Discard policy uses ack_notifiers to prevent injection of PIT interrupts
    before EOI from the last one.
    
    This patch changes the policy to always try to deliver the interrupt,
    which makes a difference when its vector is in ISR.
    Old implementation would drop the interrupt, but proposed one injects to
    IRR, like real hardware would.
    
    The old policy breaks legacy NMI watchdogs, where PIT is used through
    virtual wire (LVT0): PIT never sends an interrupt before receiving EOI,
    thus a guest deadlock with disabled interrupts will stop NMIs.
    
    Note that NMI doesn't do EOI, so PIT also had to send a normal interrupt
    through IOAPIC.  (KVM's PIT is deeply rotten and luckily not used much
    in modern systems.)
    
    Even though there is a chance of regressions, I think we can fix the
    LVT0 NMI bug without introducing a new tick policy.
    Reported-by: default avatarYuki Shibuya <shibuya.yk@ncos.nec.co.jp>
    Reviewed-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
    Signed-off-by: default avatarRadim Krčmář <rkrcmar@redhat.com>
    Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
    Signed-off-by: default avatarBen Hutchings <ben@decadent.org.uk>
    5fd34264
i8254.c 19.3 KB