HIGHMEM is broken when working in SMP V6 mode

[Nicolas Pitre]

On Thu, 27 Jan 2011, Russell King - ARM Linux wrote:

> On Thu, Jan 27, 2011 at 01:40:37PM -0500, Nicolas Pitre wrote:
> > On Thu, 27 Jan 2011, Russell King - ARM Linux wrote:
> > 
> > > On Mon, Jan 24, 2011 at 02:58:07PM -0500, Nicolas Pitre wrote:
> > > >  /*
> > > > + * The reason for kmap_high_get() is to ensure that the currently kmap'd
> > > > + * page usage count does not decrease to zero while we're using its
> > > > + * existing virtual mapping in an atomic context.  With a VIVT cache this
> > > > + * is essential to do, but with a VIPT cache this is only an optimization
> > > > + * so not to pay the price of establishing a second mapping if an existing
> > > > + * one can be used.  However, on platforms without hardware TLB maintainence
> > > > + * broadcast, we simply cannot use ARCH_NEEDS_KMAP_HIGH_GET at all since
> > > > + * the locking involved must also disable IRQs which is incompatible with
> > > > + * the IPI mechanism used by global TLB operations.
> > > > + */
> > > > +#define ARCH_NEEDS_KMAP_HIGH_GET
> > > > +#if defined(CONFIG_SMP) && defined(CONFIG_CPU_TLB_V6)
> > > > +#undef ARCH_NEEDS_KMAP_HIGH_GET
> > > > +#if defined(CONFIG_HIGHMEM) && defined(CONFIG_CPU_CACHE_VIVT)
> > > > +#error "The sum of feature in your kernel config cannot be supported together"
> > > > +#endif
> > > 
> > > This is wrong.  Take a moment to consider a kernel supporting an ARMv6
> > > VIPT aliasing cache CPU and ARMv7 SMP.  Don't we need kmap_high_get()
> > > for ARMv6 VIPT aliasing cache?
> > 
> > We don't support highmem on aliasing VIPT.  Highmem gets disabled at run 
> > time on aliasing VIPT platforms.
> 
> Correct.  But a kernel which has this configuration:
> 
> CONFIG_SMP=y
> CONFIG_HIGHMEM=y
> CONFIG_CPU_V6=y
> CONFIG_CPU_V7=y
> CONFIG_CPU_32v6=y
> CONFIG_CPU_32v7=y
> CONFIG_CPU_CACHE_V6=y
> CONFIG_CPU_CACHE_VIPT=y
> CONFIG_CPU_CACHE_V7=y
> CONFIG_CPU_CACHE_VIPT=y
> CONFIG_CPU_TLB_V6=y
> CONFIG_CPU_TLB_V7=y
> 
> This disables ARCH_NEEDS_KMAP_HIGH_GET in this kernel, which can be used on
> ARMv6 VIPT non-aliasing, VIPT aliasing, as well as SMP stuff.

Yes.

On a VIPT aliasing target, highmem gets disabled at run time.  So no 
issue there.

Otherwise, on VIPT, it is not essential to have 
ARCH_NEEDS_KMAP_HIGH_GET.

> If we don't need ARCH_NEEDS_KMAP_HIGH_GET on V6 and V7, we should just not
> enable it for those at all.

As I said in the patch comment, ARCH_NEEDS_KMAP_HIGH_GET is still a nice 
optimization on VIPT. It avoids the creation of a second mapping and TLB 
usage when an existing mapping from the pkmap array can be reused right 
away.  But if our kernel config indicates that we might run on a target 
lacking hw TLB broadcast then we have to compromize and give up on that 
optimization.


Nicolas