Re: [Qemu-devel] [PATCH v2 8/8] spapr_pci: Use XICS interrupt allocator and do not cache interrupts in PHB

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 05/21/2014 10:42 PM, Alexey Kardashevskiy wrote:
> On 05/21/2014 08:35 PM, Alexander Graf wrote:
>>
>> On 21.05.14 12:13, Alexey Kardashevskiy wrote:
>>> On 05/21/2014 07:50 PM, Alexander Graf wrote:
>>>> On 21.05.14 11:33, Alexey Kardashevskiy wrote:
>>>>> On 05/21/2014 07:13 PM, Alexander Graf wrote:
>>>>>> On 21.05.14 11:11, Michael S. Tsirkin wrote:
>>>>>>> On Wed, May 21, 2014 at 11:06:09AM +0200, Alexander Graf wrote:
>>>>>>>> On 21.05.14 10:52, Alexey Kardashevskiy wrote:
>>>>>>>>> On 05/21/2014 06:40 PM, Alexander Graf wrote:
>>>>>>>>>> On 15.05.14 11:59, Alexey Kardashevskiy wrote:
>>>>>>>>>>> Currently SPAPR PHB keeps track of all allocated MSI/MISX
>>>>>>>>>>> interrupt as
>>>>>>>>>>> XICS used to be unable to reuse interrupts which becomes a
>>>>>>>>>>> problem for
>>>>>>>>>>> dynamic MSI reconfiguration which is happening on guest driver
>>>>>>>>>>> reload or
>>>>>>>>>>> PCI hot (un)plug. Another problem is that PHB has a limit of devices
>>>>>>>>>>> supporting MSI/MSIX (SPAPR_MSIX_MAX_DEVS=32) and there is no good
>>>>>>>>>>> reason
>>>>>>>>>>> for that.
>>>>>>>>>>>
>>>>>>>>>>> This makes use of new XICS ability to reuse interrupts.
>>>>>>>>>>>
>>>>>>>>>>> This removes cached MSI configuration from SPAPR PHB so the first
>>>>>>>>>>> IRQ
>>>>>>>>>>> number
>>>>>>>>>>> of a device is stored in MSI/MSIX config space so there is no
>>>>>>>>>>> need to
>>>>>>>>>>> store
>>>>>>>>>>> this anywhere else. From now on, SPAPR PHB only keeps flags telling
>>>>>>>>>>> what
>>>>>>>>>>> type
>>>>>>>>>>> of interrupt for which device it has configured in order to return
>>>>>>>>>>> error if
>>>>>>>>>>> (for example) MSIX was enabled and the guest is trying to disable
>>>>>>>>>>> MSI
>>>>>>>>>>> which
>>>>>>>>>>> it has not enabled.
>>>>>>>>>>>
>>>>>>>>>>> This removes a limit for the maximum number of MSIX-enabled devices
>>>>>>>>>>> per PHB,
>>>>>>>>>>> now XICS and PCI bus capacity are the only limitation.
>>>>>>>>>>>
>>>>>>>>>>> This changes migration stream as it fixes
>>>>>>>>>>> vmstate_spapr_pci_msi::name
>>>>>>>>>>> which was
>>>>>>>>>>> wrong since the beginning.
>>>>>>>>>>>
>>>>>>>>>>> This fixed traces to be more informative.
>>>>>>>>>>>
>>>>>>>>>>> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
>>>>>>>>>>> ---
>>>>>>>>>>>
>>>>>>>>>>> In reality either MSIX or MSI is enabled, never both. So I could
>>>>>>>>>>> remove
>>>>>>>>>>> msi/msix
>>>>>>>>>>> bitmaps from this patch, would it make sense?
>>>>>>>>>> Is this a hard requirement? Does a device have to choose between
>>>>>>>>>> MSIX and
>>>>>>>>>> MSI or could it theoretically have both enabled? Is this a PCI
>>>>>>>>>> limitation,
>>>>>>>>>> a PAPR/XICS limitation or just a limitation of your implementation?
>>>>>>>>> My implementation does not have this limitation, I asked if I can
>>>>>>>>> simplify
>>>>>>>>> code by introducing one :)
>>>>>>>>>
>>>>>>>>> I cannot see any reason why PCI cannot have both MSI and MSIX enabled
>>>>>>>>> but
>>>>>>>>> it does not seem to be used by anyone => cannot debug and confirm.
>>>>>>>>>
>>>>>>>>> PAPR spec assumes that if the guest tries enabling MSIX when MSI is
>>>>>>>>> already
>>>>>>>>> enabled, this is a "change", not enabling both types. But it also
>>>>>>>>> says MSI
>>>>>>>>> and MSIX vector numbers are not shared. Hm.
>>>>>>>> Yeah, I'm not aware of any limitation on hardware here and I'd
>>>>>>>> rather not impose one.
>>>>>>>>
>>>>>>>> Michael, do you know of any hardware that uses MSI *and* MSI-X at
>>>>>>>> the same time?
>>>>>>>>
>>>>>>>>
>>>>>>>> Alex
>>>>>>> No, and the PCI spec says:
>>>>>>>       A function is permitted to implement both MSI and MSI-X, but
>>>>>>> system
>>>>>>>       software is
>>>>>>>       prohibited from enabling both at the same time. If system software
>>>>>>>       enables both at the same time, the result is undefined.
>>>>>> Ah, cool. So yes Alexey, feel free to impose it :).
>>>>> Heh. This solves just half of the problem - I still have to keep track of
>>>>> what device got MSI/MSIX configured via that ibm,change-msi interface. I
>>>>> was hoping I can store such flag somewhere in a device PCI config space
>>>>> but
>>>>> MSI/MSIX enable bit is not good as it is not set when those calls are
>>>>> made.
>>>>> And I cannot rely on address/data fields much as the guest can change them
>>>>> (I already use them to store IRQ numbers and btw it is missing checks when
>>>>> I read them back for disposal, I'll fix in next round).
>>>>>
>>>>> Or on "enable" event I could put IRQ numbers to .data of MSI config space
>>>>> and on "disable" check if it is not zero, then configuration took place,
>>>>> then I can remove my msi[]/msix[] flag arrays. If the guest did any change
>>>>> to MSI/MSIX config space (it does not on SPAPR except weird selftest
>>>>> cases), I compare .data with what ICS can possibly have and either reject
>>>>> "disable" or handle it and if it breaks XICS - that's too bad for the
>>>>> stupid guest. Would that be acceptable?
>>>> Can't you prohibit the guest from writing to the MSI configuration
>>>> registers itself? Then you don't need to do sanity checks.
>>>
>>> I could for emulated devices but VFIO uses the same code. For example,
>>> there is an IBM SCSI IPR card which does a "self test". For that, it saves
>>> MSIX BAR content, does reboot via some backdoor interface and restores MSIX
>>> BAR. It has been solved for VFIO in the host kernel by restoring MSIX data
>>> from cached values when guest is trying to restore it with what it thinks
>>> is actual MSIX data (it is virtualized because of x86). But there is cache
>>
>> We already have a cache because we don't access the real PCI registers with
>> msi_set_message(), no?
> 
> 
> For emulated devices there is no cache. And in any case the guest is
> allowed to write to it... Who knows what AIX does? I do not.


Tried GHashTable for keeping bus:dev.fn <-> (irq, num), more or less ok but
how to migrate such thing? Temporary cache somewhere and then unpack it? Or
use old style migration callbacks?



-- 
Alexey