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    SubjectRe: [PATCH RFC] x86/acpi: don't ignore I/O APICs just because there's no local APIC
    Jeremy Fitzhardinge <> writes:

    > On 06/17/09 19:58, Eric W. Biederman wrote:
    >>> One of the options we discussed was changing the API to get rid of the exposed
    >>> vector, and just replace it with an operation to directly bind a gsi to a pirq
    >>> (internal Xen physical interrupt handle, if you will), so that Xen ends up doing
    >>> all the I/O APIC programming internally, as well as the local APIC.
    >> As an abstraction layer I think that will work out a lot better long term.
    >> Given what iommus with irqs and DMA I expect you want something like
    >> that, that can be used from domU. Then you just make allowing the
    >> operation conditional on if you happen to have the associated hardware
    >> mapped into your domain.
    > A domU with a PCI passthrough device can bind a pirq to one of its event
    > channels. All the gsi->pirq binding happens in dom0, but binding a pirq
    > to event channel can happen anywhere (that's why it doesn't bind gsi
    > directly to event channel, as they're strictly per-domain).
    > MSI interrupts also get bound to pirqs, so once the binding is created,
    > MSI and GSI interrupts can be treated identically (I think, I haven't
    > looked into the details yet).
    >>> On the Linux side, I think it means we can just point pcibios_enable/disable_irq
    >>> to our own xen_pci_irq_enable/disable functions to create the binding between a
    >>> PCI device and an irq.
    >> If you want xen to assign the linux irq number that is absolutely the properly place
    >> to hook.
    > Yes. We'd want to keep the irq==gsi mapping for non-MSI interrupts, but
    > that's easy enough to arrange.
    >> When I was messing with the irq code I did not recall finding many
    >> cases where migrating irqs from process context worked without hitting
    >> hardware bugs. ioapic state machine lockups and the like.
    > Keir mentioned that Xen avoids masking/unmasking interrupts in the I/O
    > APIC too much, because that has been problematic in the past. Is that
    > related to the problems you're talking about? Is there anywhere which
    > documents them?

    Not in great detail. I have some comments in the code and some messages
    on the mailing list.

    What I know is that in linux the historical practice has always been
    to migrate irqs in interrupt context and in testing I found I could
    lock up ioapic state machines when I migrate interrupts from process
    context enough.

    It really cleans up the code not to migrate interrupts in the
    interrupt handler. So I spent a week or two on it.

    >> How does Xen handle domU with hardware directly mapped?
    > We call that "pci passthrough". Dom0 will bind the gsi to a pirq as
    > usual, and then pass the pirq through to the domU. The domU will bind
    > the pirq to an event channel, which gets mapped to a Linux irq and
    > handled as usual.

    Interesting. How does domU find out the pirq -> pci device mapping?

    >> Temporally ignoring what we have to do to work with Xen 3.4. I'm curious
    >> if we could make the Xen dom0 irq case the same as the Xen domU case.
    > It is already; once the pirq is prepared, the process is the same in
    > both cases.

    I 3/4 believe that. map_domain_pirq appears to setup a per domain
    mapping between the hardware vector and the irq name it is known as.
    So I don't see how that works for other domains.

    msi is setup on a per domain basis.


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