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

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Date:	Mon, 24 Nov 2014 14:45:24 -0700
From:	Alex Williamson <alex.williamson@...hat.com>
To:	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 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,

Alex


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