Re: [Xen-devel] Legacy PCI interrupt {de}assertion count

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.03.17 at 17:54, <roger.pau@xxxxxxxxxx> wrote:
> As I understand it, for level triggered legacy PCI interrupts Xen sets up a
> timer in order to perform the EOI if the guest takes too long in deasserting
> the line. This is done in pt_irq_time_out. What I don't understand is why this
> function also does a deassertion of the guest view of the PCI interrupt, ie:
> why it calls hvm_pci_intx_deassert. This AFAICT will clear the pending assert
> in the guest, and thus the guest will end up loosing one interrupt.

Especially with the comment next to the respective set_timer() it
looks to me as if this was the intended effect: If the guest didn't
care to at least start handling the interrupt within PT_IRQ_TIME_OUT,
we want it look to be lost in order to not have it block other
interrupts inside the guest (i.e. there's more to it than just
guarding the host here).

"Luckily" commit 0f843ba00c ("vt-d: Allow pass-through of shared
interrupts") introducing this has no description at all. Let's see if
Kevin remembers any further details ...

Jan


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