linux-kernel - Re: [PATCH 03/26] iommu/vt-d: Split up iommu->domains array

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Message-ID: <20150807111156.GY14980@8bytes.org>
Date:	Fri, 7 Aug 2015 13:11:56 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	iommu@...ts.linux-foundation.org,
	David Woodhouse <dwmw2@...radead.org>,
	linux-kernel@...r.kernel.org, jroedel@...e.de
Subject: Re: [PATCH 03/26] iommu/vt-d: Split up iommu->domains array

On Thu, Aug 06, 2015 at 01:20:09PM -0600, Alex Williamson wrote:
> On Wed, 2015-08-05 at 17:18 +0200, Joerg Roedel wrote:
> >  static void set_iommu_domain(struct intel_iommu *iommu, u16 did,
> >  			     struct dmar_domain *domain)
> >  {
> > -	iommu->domains[did] = domain;
> > +	struct dmar_domain **domains;
> > +	int idx = did >> 8;
> > +
> > +	if (!iommu->domains[idx]) {
> > +		size_t size = 256 * sizeof(struct dmar_domain *);
> > +		iommu->domains[idx] = kzalloc(size, GFP_ATOMIC);
> > +	}
> > +
> > +	domains = iommu->domains[idx];
> > +	if (WARN_ON(!domains))
> > +		return;
> > +	else
> > +		domains[did & 0xff] = domain;
> >  }
> 
> I'm tempted to suggest using pages here since we're dealing with 2k
> second level arrays, but maybe caring about pointers per page just makes
> that ugly.

The benefit would be that we avoid the slab-overhead while allocating.
But since the VT-d driver is needed on platforms with different
page-sizes, the handling would be indeed more complicated because we
have to split the index at a different point then depending on the
architecture.


	Joerg

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