Subject: Oops in guest after ioremap() on ARMv7

From: [email-masked] (Catalin Marinas)
Date: Tue, 3 Jan 2012 16:50:27 +0000

On Fri, Dec 23, 2011 at 12:00:25PM +0000, Ian Campbell wrote:
> On Thu, 2011-12-22 at 18:33 +0000, David Vrabel wrote:
> > On 22/12/11 18:13, Catalin Marinas wrote:
> > > On Thu, Dec 22, 2011 at 04:38:23PM +0000, David Vrabel wrote:
> > >> On 22/12/11 14:49, Catalin Marinas wrote:
> > >>> What's the value of the VTCR register for this guest? Are the
> > >>> translation table walks marked as cacheable? Also, are the page table
> > >>> attributes Normal Cacheable in the stage 2 translation? The processor
> > >>> chooses the more restrictive attribute between stage 1 and stage 2.
> > >>
> > >> VTCR = 0x80002558 which is: Outer Shareable; Normal memory, outer
> > >> write-back write-allocate cacheable; Normal memory, inner write-back,
> > >> write-allocate cacheable.

BTW, it would be better to mark the stage 2 table walks as inner
shareable, i.e. VTCR = 0x80003558. That's because in case of SMP you
would want page table accesses to snoop the caches of the other CPUs.

> > >> L3 TT entries for stage 2 have the following attributes:
> > >> Outer-Shareable; Normal, inner write-back cachable; Normal, outer
> > >> write-back cacheable.
> > >>
> > >> These look sensible to me.
> > > 
> > > They look fine (UP system). BTW, I assume that the hypervisor also
> > > flushes the caches and TLBs for the stage 2 translation tables.
> > 
> > I think so. Cc'ing Ian Campbell who knows the hypervisor side better
> > than me.
> 
> At the moment we build the entire p2m before we ever load the VTTBR or
> enable stage-2 translations in the HCR. Is that sufficient or do we also
> need to flush something?

If the model does not correctly implement cacheable page table walks for
either stage 1 or stage 2 translation (though ID_MMFR3[23:20] indicate
that it should), the hypervisor would need to clean the D-cache to the
point of unification (not necessary to go to the point of coherency)
before any of the state 2 translation tables are used.

> Obviously we will need to make sure we do appropriate flushes when we
> start needing to change the p2m of a running guest etc. Currently our
> write_pte does a flush with DCCMVAC and in general our global flushes
> are at the more aggressive end of the scale (correctness before
> optimisation ;-)).

You can use DCCMVAU but could also check the coherent walk bits in
ID_MMFR3 (assuming that the model works correctly).

-- 
Catalin