linux-kernel - Re: [PATCH 1/3] PCI/MSI: Initial hook for archs to declare multivector MSI support

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Date:	Tue, 25 Nov 2014 11:22:23 +0800
From:	Jiang Liu <jiang.liu@...ux.intel.com>
To:	Alex Williamson <alex.williamson@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
CC:	bhelgaas@...gle.com, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, x86@...nel.org, mingo@...hat.com,
	hpa@...or.com, joro@...tes.org, iommu@...ts.linux-foundation.org
Subject: Re: [PATCH 1/3] PCI/MSI: Initial hook for archs to declare multivector
 MSI support

On 2014/11/25 5:45, Alex Williamson wrote:
> On Sun, 2014-11-23 at 21:20 +0100, Thomas Gleixner wrote:
>> On Fri, 21 Nov 2014, Alex Williamson wrote:
>>> For the most part multivector MSI is not supported and drivers and
>>> hardware wanting multiple vectors opt for MSI-X instead.  It seems
>>> though that having the ability to query the arch/platform code to
>>> determine whether allocating multiple MSI vectors will ever succeed
>>> is a useful thing.  For instance, vfio-pci can use this to determine
>>> whether to expose multiple MSI vectors to the user.  If we know we
>>> cannot ever support more than one vector, we have a better shot at
>>> the userspace driver working, especially if it's a guest OS, if we
>>> only expose one vector as being available in the interface.
>>>
>>> Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
>>> ---
>>>
>>>  drivers/pci/msi.c   |    5 +++++
>>>  include/linux/msi.h |    1 +
>>>  2 files changed, 6 insertions(+)
>>>
>>> diff --git a/drivers/pci/msi.c b/drivers/pci/msi.c
>>> index 9fab30a..36b503a 100644
>>> --- a/drivers/pci/msi.c
>>> +++ b/drivers/pci/msi.c
>>> @@ -79,6 +79,11 @@ int __weak arch_setup_msi_irqs(struct pci_dev *dev, int nvec, int type)
>>>  	return 0;
>>>  }
>>>  
>>> +bool __weak arch_supports_multivector_msi(struct pci_dev *dev)
>>
>> Please not another weak arch function. We are in the process to reduce
>> them not to extend them.
>>
>> arch_supports is pretty much wrong here anyway. We are moving away
>> from arch MSI dependencies simply because it's not a arch property per
>> se.
>>
>> Multi MSI is a property of the underlying interrupt controllers and
>> there might be several MSI interrupt domains on a given system. They
>> can or cannot support multi MSI.
>>
>> The current x86 implementation is a tangled maze and Jiang is in the
>> process to distangle it completely. Until thats done x86 is not going
>> to add new ad hoc interfaces.
>>
>> Once we converted everything over to the new hierarchical irqdomains
>> we can add such an interface to the code, but for now I'm not
>> accepting anything like that into x86 msi related code.
> 
> Ok, I guess I can do some ugliness with associating the IOMMU IRQ
> remapping capability with multivector MSI support within an #ifdef x86
> block.  Gross, but I think that's as accurate as I can get w/o a hook
> through the MSI code.  Is there any target for the refactoring you
> mention?  Thanks,
Hi Alex,
	The hierarchy irqdomain framework and some preparation work
for PCI MSI refactor will be merged into v3.19, there are already in
tip tree. If thing goes smoothly, we may finish PCI MSI refactoring
on x86 platform for the merging window after 3.19, not sure 3.20 or
4.0:)
Regards!
Gerry
> 
> Alex
> 
> 
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