linux-kernel - Re: [PATCH] pci: change msi-x vector to 32bit

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Date:	Sat, 16 Aug 2008 18:09:22 -0500
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Yinghai Lu <yhlu.kernel@...il.com>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	"H. Peter Anvin" <hpa@...or.com>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Andrew Vasquez <andrew.vasquez@...gic.com>
Subject: Re: [PATCH] pci: change msi-x vector to 32bit

On Sat, 2008-08-16 at 15:17 -0700, Yinghai Lu wrote:
> On Sat, Aug 16, 2008 at 1:45 PM, James Bottomley
> <James.Bottomley@...senpartnership.com> wrote:
> >> > What I still don't quite get is the benefit of large IRQ spaces ...
> >> > particularly if you encode things the system doesn't really need to know
> >> > in them.
> >>
> >> then set nr_irqs = nr_cpu_ids * NR_VECTORS))
> >> and count down for msi/msi-x?
> >
> > No, what I mean is that msis can trip directly to CPUs, so this is an
> > affinity thing (that MSI is directly bound to that CPU now), so in the
> > matrixed way we display this in show_interrupts() with the CPU along the
> > top and the IRQ down the side, it doesn't make sense to me to encode IRQ
> > affinity in the irq number again.   So it makes more sense to assign the
> > vectors based on both the irq number and the CPU affinity so that if the
> > PCI MSI for qla is assigned to CPU4 you can reassign it to CPU5 and so
> > on.
> 
> msi-x entry index, cpu_vector, irq number...
> 
> you want to different cpus have same vector?

Obviously I'm not communicating very well.  Your apparent assumption is
that irq number == vector.  What I'm saying is that's not what we've
done for individually vectored CPU interrupts in other architectures.
In those we did (cpu no, irq) == vector.  i.e. the affinity and the irq
number identify the vector.  For non-numa systems, this is effectively
what you're interested in doing anyway.  For numa systems, it just
becomes a sparse matrix.

James


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