[PATCH] arm: tcm: Don't crash when TCM banks are protected by TrustZone

[Dave Martin]

On Thu, May 28, 2015 at 01:32:10PM +0200, Michael van der Westhuizen wrote:
> 
> > On 28 May 2015, at 12:16 PM, Dave Martin <Dave.Martin at arm.com> wrote:
> > 
> > On Thu, May 28, 2015 at 11:36:00AM +0200, Michael van der Westhuizen wrote:
> >> Fixes the TCM initialisation code to handle TCM banks that are
> >> present but inaccessible due to TrustZone configuration.  This is
> >> the default case when enabling the non-secure world.  It may also
> >> be the case that that the user decided to use TCM for TrustZone.
> >> 
> >> This change has exposed a bug in handling of TCM where no TCM bank
> >> was usable (the 0 size TCM case).  This change addresses the
> >> resulting hang.
> > 
> > The TCM registers in CP15 are not part of the architecture -- behaviour
> > is IMP DEF in v7.
> 
> My reading of DDI0406C_C is that the register is defined (CP15, c0,
> c0, 2), but the format is either v6 format or implementation defined.
> 
> The manual explicitly states that in v7 the register must be
> implemented and that when v7 format is used that the meaning of bits
> 28:0 is implementation defined (this is all in B4.1.132).
> 
> The ARM goes on to state that when no TCMs are implemented the TCMTR
> register must be implemented in ARMv6 format, indicating no TCM banks
> (i.e. all defined bits must be 0).
> 
> So, since this code assumes v6 format should I just add a check that
> bits 31:29 or 0b000?  If I do this, then my reading is that this will
> continue to work reliably in the face of v7 implementations that use
> v7 (implementation defined) format.

You're right, that looks sound.

Providing that TCMTR is read first and reports v6 format, then access
to the region registers will either succeed safely, or Undef (when
disallowed by the Secure World).

TCMTR itself is guaranteed to be readable even in the ARMv6 base
architecture.

ARMv8 gives mixed messages on this point[1], but it appears[2] that
the intention is for the above check to continue to work.

[1] DDI0487A.e, G6.2.126 (TCMTR, TCM Type Register)
[2] DDI0487A.e, G3.5 (System register support for IMPLEMENTATION DEFINED
memory features)


It would be good to see testing of this in a multiplatform kernel,
assuming you haven't tried it already.

[...]

Cheers
---Dave