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    SubjectRe: [PATCH] xen: reuse the same pirq allocated when driver load first time

    On 2013-05-15 17:41, Stefano Stabellini wrote:
    > On Tue, 14 May 2013, Konrad Rzeszutek Wilk wrote:
    >> On Tue, May 14, 2013 at 02:49:50PM +0100, Stefano Stabellini wrote:
    >>> On Mon, 13 May 2013, Konrad Rzeszutek Wilk wrote:
    >>>> On Mon, May 13, 2013 at 06:24:46PM +0100, Stefano Stabellini wrote:
    >>>>> On Mon, 13 May 2013, Konrad Rzeszutek Wilk wrote:
    >>>>>> On Mon, May 13, 2013 at 03:50:52PM +0100, Stefano Stabellini wrote:
    >>>>>>> On Mon, 13 May 2013, Konrad Rzeszutek Wilk wrote:
    >>>>>>>> On Mon, May 13, 2013 at 12:06:43PM +0100, Stefano Stabellini wrote:
    >>>>>>>>> On Fri, 10 May 2013, Konrad Rzeszutek Wilk wrote:
    >>>>>>>>>> On Wed, May 08, 2013 at 04:18:24PM +0800, Zhenzhong Duan wrote:
    >>>>>>>>>>> When driver load and unload in a loop, pirq will exhaust finally.
    >>>>>>>>>>> Try to use the same pirq which was already mapped and binded at first time
    >>>>>>>>>> So what happens if I unload and reload two drivers in random order?
    >>>>>>>>>>
    >>>>>>>>>>> when driver loaded.
    >>>>>>>>>>>
    >>>>>>>>>>> Read pirq from msix entry and test if data is XEN_PIRQ_MSI_DATA
    >>>>>>>>>>> xen_irq_from_pirq(pirq) < 0 checking is wrong as irq will be freed
    >>>>>>>>>>> when driver unload, it's always true in second load.
    >>>>>>>>>> If my understanding is right the issue at hand is that the caching
    >>>>>>>>>> information about the pirq disappears once the driver has been
    >>>>>>>>>> unloaded b/c the event's irq-info is removed (as the driver is
    >>>>>>>>>> unloaded and free_irq is called).
    >>>>>>>>>>
    >>>>>>>>>> Stefano,
    >>>>>>>>>> Is there a specific write to the MSI structure that would cause the
    >>>>>>>>>> hypervisor to drop the PIRQ? Or a nice hypercall to "free" an
    >>>>>>>>>> PIRQ in usage?
    >>>>>>>>> We already have a "free PIRQ" hypercall, it's called
    >>>>>>>>> PHYSDEVOP_unmap_pirq and should be called by QEMU.
    >>>>>>>> Considering that we call function that allocates (PHYSDEVOP_get_free_pirq)
    >>>>>>>> it in the Linux kernel (and not in QEMU), perhaps that should be done in the
    >>>>>>>> Linux kernel as part of xen_destroy_irq()? Or would that confuse QEMU?
    >>>>>>> I think it would confuse QEMU. It is probably better to let the unmap
    >>>>>>> being handled by it.
    >>>>>>>
    >>>>>>>
    >>>>>>>> It looks like QEMU only does that hypercall (via xc_physdev_unmap_pirq)
    >>>>>>>> unregister_real_device which is only called during pci unplug?
    >>>>>>> You are right! I would think that this behaviour is erroneous unless it
    >>>>>>> was done on purpose to avoid allocating MSIs twice.
    >>>>>>> If that is the case we would need to do something similar in Linux too.
    >>>>>>>
    >>>>>>> I think that the issue is the mismatch between QEMU's and Linux's
    >>>>>>> behaviours: either both should be allocating MSIs once, or they should
    >>>>>>> both be allocating and deallocating MSIs every time the driver is loaded
    >>>>>>> and unloaded.
    >>>>>> Right. But we also have the scenario that QEMU and Linux are going to
    >>>>>> be out of sync. So we need fixes in both places - I think.
    >>>>> QEMU is the owner of the pirq, in fact it is the one that creates and
    >>>>> destroys the mapping. I think that the right place to fix this problem
    >>>>> is in QEMU, the ABI would be much cleaner as a result. As a side effect
    >>>>> we don't need to make any changes in Linux.
    >>>> You do. You need to remove the PHYSDEVOP_get_free_pirq call in that case.
    >>>
    >>> PHYSDEVOP_get_free_pirq needs to stay, because Linux needs to know the
    >>> pirq that QEMU is going to use.
    >> That looks like an API violation. We have an hypercall that
    >> allocates the PIRQ in the Linux, then two hypercalls in the QEMU
    >> layer - one to map, and the other to unmap and free.
    >>
    >>> However I would let QEMU handle the mapping (it already does that in
    >>> pt_msi_setup calling xc_physdev_map_pirq_msi) and unmapping (that is
    >>> done by calling xc_domain_unbind_msi_irq from pt_msi_disable).
    >>> I think the problem is that pt_msi_disable is only called on
    >>> unregister_real_device and pt_reset_interrupt_and_io_mapping, not when
    >>> the guest disables MSIs.
    >> Sure, I am not disputing that. I think the fix in QEMU to call the
    >> unmap is correct.
    >>
    >> But I am also wondering whether it makes sense to do that in the Linux
    >> kernel - as it does the alloc in the first place. Seems like a bit of
    >> duct-tape has been used to connect this plumbing together.
    >
    > I admit that it is not a great interface.
    > I would be open to options that move the entire setup/freeing in Linux,
    > but keep in mind that we need to retain the pirq code in QEMU for pure
    > HVM guests.
    Hi Stefano,

    do you work out a patch for me to test?

    thanks
    zduan


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