linux-kernel - Re: [PATCH] iommu/arm-smmu-v3: avoid over allocating for l2 stream tables

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Message-ID: <20161220102209.GC10132@arm.com>
Date:   Tue, 20 Dec 2016 10:22:09 +0000
From:   Will Deacon <will.deacon@....com>
To:     Nate Watterson <nwatters@...eaurora.org>
Cc:     Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iommu/arm-smmu-v3: avoid over allocating for l2 stream
 tables

Hi Nate,

On Mon, Dec 19, 2016 at 03:26:40PM -0500, Nate Watterson wrote:
> Currently, all l2 stream tables are being allocated with space for
> (1<<split) stes without regard to the number of sid bits the smmu
> physically supports. To avoid allocating memory for inaccessible
> stes, this patch limits the span of an l2 table to be no larger
> than the sid size of the smmu to which it belongs.
> 
> Signed-off-by: Nate Watterson <nwatters@...eaurora.org>
> ---
>  drivers/iommu/arm-smmu-v3.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)

I can't help but think you'd be better off using a linear stream table
in this scenario. If we hack the feature check for
ARM_SMMU_FEAT_2_LVL_STRTAB so that it doesn't report support for 2 level
tables if the number of sids is less than that covered by a single l2
entry, would that solve your problem?

Will