Prevent IRQ sharing for PCI passthrough

Prevent IRQ sharing for PCI passthrough

[Dirk Gouders]

Hello List,

please don't shout at me if the answer to my question is too obvious,
but in the German Linux Magazine I read an article about passing PCI
devices to KVM guests and the authors said that "shared interrupt
funcionality has to be disabled" which I read as "it is possible to
disable shared interrupt functionality".

I currently have the problem with a PCI ISDN card that is not MSI
capable and even if I change the PCI slot and disable all other slots,
USB etc. in the BIOS, that card still shares its interrupt with three
other devices.

Even after extensive searching/reading, I am not sure if it possible to
prevent a PCI card from sharing an interrupt with other devices or
probably manually assign a free interrupt to that card via some kernel
parameter and would be glad if someone could give me an answer to that
question.

Best regards,

Dirk

Re: Prevent IRQ sharing for PCI passthrough

[Avi Kivity]

On 05/13/2010 07:57 PM, Dirk Gouders wrote:
> Hello List,
>
> please don't shout at me if the answer to my question is too obvious,
>    

We won't shout at you.

> but in the German Linux Magazine I read an article about passing PCI
> devices to KVM guests and the authors said that "shared interrupt
> funcionality has to be disabled" which I read as "it is possible to
> disable shared interrupt functionality".
>    

It would be more correct to say "no go with shared host interrupts".

> I currently have the problem with a PCI ISDN card that is not MSI
> capable and even if I change the PCI slot and disable all other slots,
> USB etc. in the BIOS, that card still shares its interrupt with three
> other devices.
>
> Even after extensive searching/reading, I am not sure if it possible to
> prevent a PCI card from sharing an interrupt with other devices or
> probably manually assign a free interrupt to that card via some kernel
> parameter and would be glad if someone could give me an answer to that
> question.
>    

There is an ACPI _SRS method which can be used to move interrupts 
around.  However Linux doesn't appear to expose it.  Even if it did, the 
interrupt may be shared on the motherboard, in which case nothing would 
help (though you might be able to share it with unused devices).

Chris?

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

Re: Prevent IRQ sharing for PCI passthrough

[Chris Wright]

* Avi Kivity (avi@redhat.com) wrote:
> On 05/13/2010 07:57 PM, Dirk Gouders wrote:
> >Hello List,
> >
> >please don't shout at me if the answer to my question is too obvious,
> 
> We won't shout at you.
> 
> >but in the German Linux Magazine I read an article about passing PCI
> >devices to KVM guests and the authors said that "shared interrupt
> >funcionality has to be disabled" which I read as "it is possible to
> >disable shared interrupt functionality".
> 
> It would be more correct to say "no go with shared host interrupts".
> 
> >I currently have the problem with a PCI ISDN card that is not MSI
> >capable and even if I change the PCI slot and disable all other slots,
> >USB etc. in the BIOS, that card still shares its interrupt with three
> >other devices.
> >
> >Even after extensive searching/reading, I am not sure if it possible to
> >prevent a PCI card from sharing an interrupt with other devices or
> >probably manually assign a free interrupt to that card via some kernel
> >parameter and would be glad if someone could give me an answer to that
> >question.
> 
> There is an ACPI _SRS method which can be used to move interrupts
> around.  However Linux doesn't appear to expose it.  Even if it did,
> the interrupt may be shared on the motherboard, in which case
> nothing would help (though you might be able to share it with unused
> devices).

The only way to work around this is to unbind the other drivers that
are sharing that interrupt.  This may not work if the other devices are
critical to you system (it's typically sharing w/ USB Host Controller).

Otherwise, we can't support this mode easily (there is some work in
progress to allow this to work if your ISDN device is PCI 2.3
compliant).

thanks,
-chris

Re: Prevent IRQ sharing for PCI passthrough

[Dirk Gouders]

>> Even after extensive searching/reading, I am not sure if it possible to
>> prevent a PCI card from sharing an interrupt with other devices or
>> probably manually assign a free interrupt to that card via some kernel
>> parameter and would be glad if someone could give me an answer to that
>> question.
>>    
>
> There is an ACPI _SRS method which can be used to move interrupts
> around.  However Linux doesn't appear to expose it.  Even if it did,
> the interrupt may be shared on the motherboard, in which case nothing
> would help (though you might be able to share it with unused devices).

You mean by providing a custom DSDT, right?
A quick look in drivers/acpi/pci_link.c shows a TBD comment mentioning
_SRS but I will have to have a deeper look at this subject, because its
quite new to me.

Thanks for your answer,

Dirk

Re: Prevent IRQ sharing for PCI passthrough

[Dirk Gouders]

>> There is an ACPI _SRS method which can be used to move interrupts
>> around.  However Linux doesn't appear to expose it.  Even if it did,
>> the interrupt may be shared on the motherboard, in which case
>> nothing would help (though you might be able to share it with unused
>> devices).
>
> The only way to work around this is to unbind the other drivers that
> are sharing that interrupt.  This may not work if the other devices are
> critical to you system (it's typically sharing w/ USB Host Controller).

Thanks for the hint.  I will try if I can produce a situation where the
card shares it's interrupt with unused devices.

Dirk

Re: Prevent IRQ sharing for PCI passthrough

[Avi Kivity]

On 05/14/2010 12:37 PM, Dirk Gouders wrote:
>    
>>> Even after extensive searching/reading, I am not sure if it possible to
>>> prevent a PCI card from sharing an interrupt with other devices or
>>> probably manually assign a free interrupt to that card via some kernel
>>> parameter and would be glad if someone could give me an answer to that
>>> question.
>>>
>>>        
>> There is an ACPI _SRS method which can be used to move interrupts
>> around.  However Linux doesn't appear to expose it.  Even if it did,
>> the interrupt may be shared on the motherboard, in which case nothing
>> would help (though you might be able to share it with unused devices).
>>      
> You mean by providing a custom DSDT, right?
> A quick look in drivers/acpi/pci_link.c shows a TBD comment mentioning
> _SRS but I will have to have a deeper look at this subject, because its
> quite new to me.
>
>    

No, there should be an _SRS method in your existing DSDT.  But Linux 
doesn't call it (except after resume etc.) and doesn't allow the user to 
use _SRS to move interrupts around.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.