Re: [Xen-devel] [PATCH v4 0/3] Set SMMU s2 input-size based on p2m tables

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Wed, 2015-05-06 at 13:26 +1000, Edgar E. Iglesias wrote:
> On Tue, May 05, 2015 at 02:17:47PM +0100, Ian Campbell wrote:
> > On Fri, 2015-05-01 at 11:40 +1000, Edgar E. Iglesias wrote:
> > > From: "Edgar E. Iglesias" <edgar.iglesias@xxxxxxxxxx>
> > > 
> > > Hi,
> > > 
> > > This is a fix for the issue I'm seeing on ZynqMP with missmatched
> > > setup of the SMMU and the shared p2m page-tables with the CPU.
> > 
> > Looking back at previous conversations it seems like your SMMU handles
> > fewer input bits than the second stage of the regular MMU, is that
> > right?
> 
> It's the other way around. The SMMU handles up-to 48 bits S2 input addrs
> but the Cortex-A53 only does 40bit IPAs.

Oops, I read backwards, thanks.

Thinking about it some more I think more (at at least as) important as
the input IPA size is the page table format, specifically the starting
level (TCR.SL0) and the amount of concatenation at the root level (I
forget how that is specified I think it is implicit in the TCR.SL0 and
TCR.T0SZ?). If we are sharing page tables then they really ought to
match.

That said I think your patch is a good start and rules out issues on at
least one axis, so it's worth doing.

> > Is there an architectural constraint that bits(SMMU) <= bits(MMU-s2)?
> 
> I'm not aware of any such constraints. In XEN, because we share page-tables
> between CPU and SMMUs, we need to make sure the SMMUs support the
> page-tables format and get configured accordingly.

Right, it seems like we may eventually need to introduce the possibility
of not sharing the p2m depending on the circumstances as is done on x86.

Ian.


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