svn commit: r245147 - head/sys/arm/include

[Oleksandr Tymoshenko]

On 1/7/2013 7:00 PM, Konstantin Belousov wrote:
> On Tue, Jan 08, 2013 at 02:40:20AM +0000, Oleksandr Tymoshenko wrote:
>> Author: gonzo
>> Date: Tue Jan  8 02:40:20 2013
>> New Revision: 245147
>> URL: http://svnweb.freebsd.org/changeset/base/245147
>>
>> Log:
>>    Switch default cache type for ARMv6/ARMv7 from write-through to
>>    writeback-writeallocate
> Could you, please, describe how this is supposed to work.
>
> Assume that a process mapped the same file page at two different
> addresses, both read-write, and writes to both mappings. Usermode code
> does not consider the need to flush caches or synchronize writes into
> non-overlapping regions, usually. Would it break, i.e. could the values
> appear in the page which were never written to it ?

I might misunderstood question so let me rephrase it:
One physical page P, virtual addresses A and B both mapped to it. Two 
conditions should
be true:

-  if I write word to A+0x200  same value should appear at B+0x200 next 
time it is read
- If there are no writes to P either through A or B each next read 
should yield same result.

These conditions are met for ARMv7 devices for both WT and WBWA caches. 
They're PIPT
so no aliasing in this case. Up until now I believed that "no aliasing" 
is true for all ARM CPUs
we target but quick check proved me wrong: ARM1176 on which Raspberry Pi
is based is prone to cache aliasing problem. Which might explain memory 
corruption
easily reproducible under load. Again the problem is not related to 
cache type itself but
to the lack of handling of this situation in pmap module.

Some info on subject:
http://blogs.arm.com/software-enablement/716-page-colouring-on-armv6-and-a-bit-on-armv7/

Thank you for raising this topic. I hope people  more ARM-savvy then me 
can confirm or refute
my point of view.

>
> Another similar question, sf buffers on ARM flush the cache on sf_buf_free()
> (sometimes). If the page, for which the sf buffer is created, mapped into
> the userspace read-write, could it be that a value appears that was never
> written ? Note that sf buffers are used e.g. by uiomove_fromphys().
I believe the stuff above is applicable to this question too.