On Sat, Oct 17, 2015 at 10:30:20PM +0200, Christoffer Dall wrote: > We have an interesting issue when the guest disables the timer interrupt > on the VGIC, which happens when turning VCPUs off using PSCI, for > example. > > The problem is that because the guest disables the virtual interrupt at > the VGIC level, we never inject interrupts to the guest and therefore > never mark the interrupt as active on the physical distributor. The > host also never takes the timer interrupt (we only use the timer device > to trigger a guest exit and everything else is done in software), so the > interrupt does not become active through normal means. > > The result is that we keep entering the guest with a programmed timer > that will always fire as soon as we context switch the hardware timer > state and run the guest, preventing forward progress for the VCPU. > > Since the active state on the physical distributor is really part of the > timer logic, it is the job of our virtual arch timer driver to manage > this state. > > The timer->map->active boolean field indicates whether we have signalled > this interrupt to the vgic and if that interrupt is still pending or > active. As long as that is the case, the hardware doesn't have to > generate physical interrupts and therefore we mark the interrupt as > active on the physical distributor. > > Cc: Marc Zyngier <marc.zyngier@xxxxxxx> > Reported-by: Lorenzo Pieralisi <lorenzo.pieralisi@xxxxxxx> > Signed-off-by: Christoffer Dall <christoffer.dall@xxxxxxxxxx> > --- Marc was worried about the performance implications of this fix on Mustang given the potentially slow MMIO path to the GIC on that system, so I ran some before and after applying this series: BM Hackbench Kernbench PbZip C PbZip D -- --------- --------- ------- ------- Before 17.94 51.66 17.69 10.59 After 18.14 51.62 17.82 10.62 The slight increase on hackbench is well within the variability (1.409 for the 8 runs behind these numbers) so I don't think this will be noticable. That said, there's room for optimizations here by only touching the GIC on vcpu load/put and when the value changes, but I think this is premature. -Christoffer -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
