[PATCH v3 4/6] arm: Add icache invalidation on switch_mm for Cortex-A15

[Marc Zyngier]

On Sat, 27 Jan 2018 14:23:57 -0800
Florian Fainelli <f.fainelli at gmail.com> wrote:

> On 01/25/2018 07:21 AM, Marc Zyngier wrote:
> > In order to avoid aliasing attacks against the branch predictor,
> > Cortex-A15 require to invalidate the BTB when switching
> > from one user context to another. The only way to do so on this
> > CPU is to perform an ICIALLU, having set ACTLR[0] to 1 from secure
> > mode.  
> 
> Even though this is a platform design mistake, let's say your Linux
> kernel boots in secure supervisor mode, we could have code
> that tries to set ACTLR[0] as early as possible, since the writes are
> ignored if executing from non-secure mode. If Linux is booted normally
> either PL1 or PL2 non-secure we could still check ACTLR[0].
> 
> My concern is that without doing this, we may have a hard time catching
> improper firmware as well as having bogus bug reports. This is
> completely RFC though since:
> 
> - I could not quite figure out yet why update ca15_actlr_status from
> assembly is not reflected in the C code despite using PC relative loads

One potential problem is that you're writing this with the MMU off, and
reading it with the MMU on. Unless you've added CMOs to make this
visible, it will not be reliable.

Also, I'm not sure how this works with MCPM.

> - this is done in __ca15mp_setup and __b15mp_setup because we know by
> then that we have either of these two CPUs but we could presumably move
> this check under a Kconfig option and earlier where all erratas are checked

Thanks,

	M.
-- 
Without deviation from the norm, progress is not possible.