linux-kernel - Re: [Patch Part2 v3 15/24] x86, MSI: Use hierarchy irqdomain to manage MSI interrupts

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Date:	Fri, 31 Oct 2014 15:00:50 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jiang Liu <jiang.liu@...ux.intel.com>
cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	Yinghai Lu <yinghai@...nel.org>,
	Borislav Petkov <bp@...en8.de>,
	Grant Likely <grant.likely@...aro.org>,
	Marc Zyngier <marc.zyngier@....com>,
	Yingjoe Chen <yingjoe.chen@...iatek.com>, x86@...nel.org,
	Joerg Roedel <joro@...tes.org>,
	Matthias Brugger <matthias.bgg@...il.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tony Luck <tony.luck@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	linux-acpi@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	iommu@...ts.linux-foundation.org
Subject: Re: [Patch Part2 v3 15/24] x86, MSI: Use hierarchy irqdomain to
 manage MSI interrupts

On Fri, 31 Oct 2014, Jiang Liu wrote:
> On 2014/10/29 5:37, Thomas Gleixner wrote:
> > Then it calls down the domain allocation chain. x86_msi_domain would
> > simply hand down to the parent domain. That would either be the remap
> > domain or the vector domain.
> The issue here is that, the hierarchy irqdomain maintains a tree
> topology and every irqdomain only supports one parent.
> 
> In case of irq remapping, we need to build one irqdomain for each IOMMU
> unit to support hotplug and simplify the implementation. So we need to
> build one MSI irqdomain for each IOMMU unit too instead of using a
> common MSI irqdomain.

That makes indeed a difference.
 
> Current design is that, a common MSI irqdomain to support all MSI when
> irq remapping is disabled, and one MSI irqdomain for each IOMMU unit
> when irq remapping is enabled.

> So we have the code below to choose the correct irqdomain for MSI.
>         domain = irq_remapping_get_irq_domain(&info);
>         if (domain == NULL)
>                 domain = msi_default_domain;
>         if (domain == NULL)
>                 return -ENOSYS;

Right. I guess we need to keep it that way for now.

But looking at the code makes me wonder why we actually need to call
into the remap code and do a list walk to figure the domain out. The
association of device and iommu should be known at startup/hotplug
time already. That's out of the scope of this work, but should be
fixed eventually.

Thanks,

	tglx




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