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

[Alexey Kardashevskiy]

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 <address@hidden>
>>>>>> ---
>>>>>>
>>>>>> 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?


-- 
Alexey