linux-kernel - Re: [PATCH 3/5] iommu: Limit iommu_attach/detach

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Message-ID: <20150130122814.GC3702@8bytes.org>
Date:	Fri, 30 Jan 2015 13:28:14 +0100
From:	Joerg Roedel <joro@...tes.org>
To:	Will Deacon <will.deacon@....com>
Cc:	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
	Kukjin Kim <kgene@...nel.org>,
	David Woodhouse <dwmw2@...radead.org>,
	Heiko Stuebner <heiko@...ech.de>,
	Hiroshi Doyu <hdoyu@...dia.com>,
	Thierry Reding <thierry.reding@...il.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	Arnd Bergmann <arnd@...db.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Robin Murphy <Robin.Murphy@....com>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	"jroedel@...e.de" <jroedel@...e.de>
Subject: Re: [PATCH 3/5] iommu: Limit iommu_attach/detach_device to devices
 with their own group

On Wed, Jan 28, 2015 at 02:35:24PM +0000, Will Deacon wrote:
> On Tue, Jan 27, 2015 at 12:08:57AM +0000, Joerg Roedel wrote:
> > @@ -51,6 +51,7 @@ struct iommu_group {
> >  	void (*iommu_data_release)(void *iommu_data);
> >  	char *name;
> >  	int id;
> > +	unsigned dev_cnt;
> 
> Is this actually used on a fast path, or can we just inspect the list of
> devices on the group instead?

Not really a fast path, but we have to hold the group mutex while
traversing the list, which could hurt performance somewhere else. Are
these 4 bytes a problem?


	Joerg

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