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

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Monday, March 27, 2017 4:00 PM
> 
> >>> 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 ...
> 

Sorry I don't remember more detail other than existing comments.
Roger, did you encounter a problem now?

Thanks
Kevin

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