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Re: [Qemu-devel] [RFC PATCH] PCI: Introduce INTx check & mask API

From: Jan Kiszka
Subject: Re: [Qemu-devel] [RFC PATCH] PCI: Introduce INTx check & mask API
Date: Thu, 24 May 2012 23:29:56 -0300
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On 2012-05-24 22:18, Alexey Kardashevskiy wrote:
> On 24/05/12 22:02, Jan Kiszka wrote:
>> On 2012-05-24 04:44, Alexey Kardashevskiy wrote:
>>> [Found while debugging VFIO on POWER but it is platform independent]
>>> There is a feature in PCI (>=2.3?) to mask/unmask INTx via PCI_COMMAND and
>>> PCI_STATUS registers.
>> Yes, 2.3 introduced this. Masking is done via command register, checking
>> if the source was the PCI in question via the status register. The
>> latter is important for supporting IRQ sharing - and that's why we
>> introduced this masking API to the PCI layer.
> Is not it just a quite small optimization to not to disable interrupts on all 
> devices which share
> the same IRQ but just on those who fired an interrupt? If so, do PCI devices 
> really often share
> IRQs? Does not supporting this mean real slowdown on such devices?
> As far as I understand, everyone who cares about performance uses MSI/MSIX, 
> no?

Not everyone is blessed with MSI-only PCI devices. From my notebook:

# cat /proc/interrupts
 22: [...] IO-APIC-fasteoi   ehci_hcd:usb1, ehci_hcd:usb2

So, if I want to assign one EHCI controller to a guest, I have to
disable the other as well. The same can happen quickly if you attach a
few legacy PCI adapters to a system and want to pass them through.

>>> And there is some API to support that (commit 
>>> a2e27787f893621c5a6b865acf6b7766f8671328).
>>> I have a network adapter:
>>> 0001:00:01.0 Ethernet controller: Chelsio Communications Inc T310 10GbE 
>>> Single Port Adapter
>>>     Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr+ 
>>> Stepping- SERR+ FastB2B- DisINTx-
>>>     Status: Cap+ 66MHz- UDF- FastB2B- ParErr- DEVSEL=fast >TAbort- <TAbort- 
>>> <MAbort- >SERR- <PERR- INTx-
>>> pci_intx_mask_supported() reports that the feature is supported for this 
>>> adapter
>>> BUT the adapter does not set PCI_STATUS_INTERRUPT so 
>>> pci_check_and_set_intx_mask()
>>> never changes PCI_COMMAND and INTx does not work on it when we use it as 
>>> VFIO-PCI device.
>>> If I remove the check of this bit, it works fine as it is called from an 
>>> interrupt handler and
>>> Status bit check is redundant.
>>> Opened a spec:
>>> PCI LOCAL BUS SPECIFICATION, REV. 3.0, Table 6-2: Status Register Bits
>>> ===
>>> 3   This read-only bit reflects the state of the interrupt in the
>>> device/function. Only when the Interrupt Disable bit in the command
>>> register is a 0 and this Interrupt Status bit is a 1, will the
>>> device’s/function’s INTx# signal be asserted. Setting the Interrupt
>>>    Disable bit to a 1 has no effect on the state of this bit.
>>> ===
>>> With this adapter, INTx# is asserted but Status bit is still 0.
>>> Is it mandatory for a device to set Status bit if it supports INTx masking?
>>> 2 Alex: if it is mandatory, then we need to be able to disable pci_2_3 in 
>>> somehow.
>> Since PCI 2.3, this bit is mandatory, and it should be independent of
>> the masking bit. The question is, if your device is supposed to support
>> 2.3, thus is just buggy, or if our detection algorithm is unreliable. It
>> basically builds on the assumption that, if we can flip the mask bit,
>> the feature should be present. I guess that is the best we can do. Maybe
>> we can augment this with a blacklist of devices that "support" flipping
>> without actually providing the feature.
> It is a good moment to start :)
> Not sure where - in VFIO or along with that PCI INTx API.

At PCI level as the API is VFIO agnostic (it was introduced for
"classic" KVM device assignment, in fact).

> Here is that broken device:
> address@hidden:~$ lspci -s 1:1:0.0
> 0001:01:00.0 Ethernet controller: Chelsio Communications Inc T310 10GbE 
> Single Port Adapter
> address@hidden:~$ lspci -ns 1:1:0.0
> 0001:01:00.0 0200: 1425:0030

A patch to add the infrastructure as well would be even more welcome. :)
You could have a look at drivers/pci/quirks.c for patterns how to do this.


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