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

[Zhenzhong Duan <zhenzhong.duan@oracle.com>]

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.
It seems MAP_PIRQ_TYPE_MSI is unsupported in physdev_hvm_map_pirq yet.
Move setup/freeing in Linux need to add that support first.

zduan