Re: [PATCH] KVM: IOAPIC: Fix level-triggered irq injection hang

[Avi Kivity <avi@qumranet.com>]

Mark McLoughlin wrote:
> The "remote_irr" variable is used to indicate an interrupt
> which has been received by the LAPIC, but not acked.
>
> In our EOI handler, we unset remote_irr and re-inject the
> interrupt if the interrupt line is still asserted.
>
> However, we do not set remote_irr here, leading to a
> situation where if kvm_ioapic_set_irq() is called, then we go
> ahead and call ioapic_service(). This means that IRR is
> re-asserted even though the interrupt is currently in service
> (i.e. LAPIC IRR is cleared and ISR/TMR set)
>
> The issue with this is that when the currently executing
> interrupt handler finishes and writes LAPIC EOI, then TMR is
> unset and EOI sent to the IOAPIC. Since IRR is now asserted,
> but TMR is not, then when the second interrupt is handled,
> no EOI is sent and if there is any pending interrupt, it is
> not re-injected.
>
> This fixes a hang only seen while running mke2fs -j on an
> 8Gb virtio disk backed by a fully sparse raw file, with
> aliguori "avoid fragmented virtio-blk transfers by copying"
> changes.
>
>   

Good catch indeed; applied.  I think it also fixes the case where the 
ioapic entry is masked when the eoi occurs.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.