2017-11-05 21:52 GMT+08:00 Nikita Leshenko <nikita.leshchenko@xxxxxxxxxx>:
> Some OSes (Linux, Xen) use this behavior to clear the Remote IRR bit for
> IOAPICs without an EOI register. They simulate the EOI message manually
> by changing the trigger mode to edge and then back to level, with the
> entry being masked during this.
>
> QEMU implements this feature in commit ed1263c363c9
> ("ioapic: clear remote irr bit for edge-triggered interrupts")
>
> As a side effect, this commit removes an incorrect behavior where Remote
> IRR was cleared when the redirection table entry was rewritten. This is not
> consistent with the manual and also opens an opportunity for a strange
> behavior when a redirection table entry is modified from an interrupt
> handler that handles the same entry: The modification will clear the
> Remote IRR bit even though the interrupt handler is still running.
>
> Signed-off-by: Nikita Leshenko <nikita.leshchenko@xxxxxxxxxx>
> Reviewed-by: Liran Alon <liran.alon@xxxxxxxxxx>
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Reviewed-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> ---
> arch/x86/kvm/ioapic.c | 11 ++++++++++-
> 1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/ioapic.c b/arch/x86/kvm/ioapic.c
> index 6df150eaaa78..163d340ee5f8 100644
> --- a/arch/x86/kvm/ioapic.c
> +++ b/arch/x86/kvm/ioapic.c
> @@ -304,8 +304,17 @@ static void ioapic_write_indirect(struct kvm_ioapic *ioapic, u32 val)
> } else {
> e->bits &= ~0xffffffffULL;
> e->bits |= (u32) val;
> - e->fields.remote_irr = 0;
> }
> +
> + /*
> + * Some OSes (Linux, Xen) assume that Remote IRR bit will
> + * be cleared by IOAPIC hardware when the entry is configured
> + * as edge-triggered. This behavior is used to simulate an
> + * explicit EOI on IOAPICs that don't have the EOI register.
> + */
> + if (e->fields.trig_mode == IOAPIC_EDGE_TRIG)
> + e->fields.remote_irr = 0;
> +
> mask_after = e->fields.mask;
> if (mask_before != mask_after)
> kvm_fire_mask_notifiers(ioapic->kvm, KVM_IRQCHIP_IOAPIC, index, mask_after);
> --
> 2.13.3
>
