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

From: Alexey Kardashevskiy
Subject: Re: [Qemu-devel] [RFC PATCH] PCI: Introduce INTx check & mask API
Date: Fri, 25 May 2012 11:18:43 +1000
User-agent: Mozilla/5.0 (X11; Linux i686; rv:11.0) Gecko/20120327 Thunderbird/11.0.1

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?

>> 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.

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


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