svn commit: r318021 - in head/sys/arm: arm include

[Andrew Turner]

> On 9 May 2017, at 13:40, Michal Meloun <melounmichal at gmail.com> wrote:
> 
> 
> 
> On 09.05.2017 13:34, Andrew Turner wrote:
>>> On 9 May 2017, at 12:05, Michal Meloun <mmel at FreeBSD.org> wrote:
>>> 
>>> Author: mmel
>>> Date: Tue May  9 11:05:32 2017
>>> New Revision: 318021
>>> URL: https://svnweb.freebsd.org/changeset/base/318021
>>> 
>>> Log:
>>> Introduce pmap_remap_vm_attr(),
>>> it allows to remap one VM memattr class to another.
>>> 
>>> This function is intent to be used as workaround for various SoC bugs,
>>> mainly access ordering/sequencing related bugs in crossbar fabric.
>> This seems quite heavy handed to change the attribute for all memory of a given type.
> Yes, exactly.  See comment in D10218 -
> /*
> * Workaround for Marvell Armada38X family HW issue
> * between Cortex-A9 CPUs and on-chip devices that may
> * cause hang on heavy load.
> * To avoid that, map all registers including PCIe IO
> * as strongly ordered instead of device memory.
> */

I don’t think it’s been answered if this is just for PCIe, or all devices.

> 
>> Other architectures have pmap_change_attr to change the attribute on a specific range of memory.
> Right. Problem is that I don't known any method how we can change 
> memory attribute for live memory in SMP system,
> without hitting undefined behavior.

I would expect drivers to change the attributes early, before they access the memory. We could also use smp_rendezvous to ensure nothing else is running, this will have a performance code, however I would not expect pmap_change_attr to be used in the fast path.

Andrew