some question about TCR setting

[Mark Rutland]

Hi,

The below applies to ARMv8 (and also ARMv7). Prior versions of the ARM
architecture do not provide the same guarantees.

On Sun, Nov 16, 2014 at 04:52:11PM +0000, Russell King - ARM Linux wrote:
> On Sun, Nov 16, 2014 at 06:53:57PM +0800, vichy wrote:
> > hi Mark:
> > 
> > > Consider CONFIG_HIGHPTE. The physical address space can be larger than
> > > the virtual address space, in which case not everything can be
> > > permanently mapped.
> > if those page tables are not mapped into the virtual address space,
> > how OS create them?
> > (AFAIK, once System.M is enabled, the processor cannot access where
> > page table doesn't map to)
> > except those page tables are created before System.M enabled.
> 
> The OS temporarily maps the page tables into virtual space to update
> them.  Once updated, the OS unmaps the page tables.
> 
> However, forget this detail - the MMU page table walker knows nothing
> about the virtual address space: MMU page table walks do not happen in
> the virtual address space, they happen in the physical address space.
> So, it makes no odds what so ever whether the table is mapped or not.
> 
> > BTW, why we need to set page table walk attribute as sharable and
> > inner/outer cacheable?
> 
> The access type is included on the bus along with the address and other
> attributes.  This includes whether it's sharable, and the cache attributes.
> A MMU page walker may be implemented such that it is capable of accessing
> L2 cache.

Or L1, depending on the implementation of the memory system. The logic
which walks the page tables is essentially another coherent observer, so
it doesn't necessarily matter precisely where it is attached.

> In that case, we would want the MMU page walker to be able to read from
> the L2 cache, which would need the access to be marked as "normal memory,
> cacheable" so that the cache is searched for a matching line.  This in
> turn means that software does not have to flush page table updates all
> the way back to the physical memory - merely flushing them out of the L1
> cache is sufficient.

To further this point, provided the TCR is programmed with the same
attributes the kernel uses to access the page tables, no cache
maintenance is necessary, as the CPU(s) and page table walker(s) are
coherent.

Barriers are necessary to ensure visibility of any page table
modifications prior to explicit memory accesses and/or TLB maintenance.

This is what we do for ARMv7 and arm64.

Thanks,
Mark.