Re: ioremap_uc() followed by set_memory_wc() - burrying MTRR

Andy Lutomirski
2015-04-22T16:53:03Z

On Wed, Apr 22, 2015 at 8:23 AM, Luis R. Rodriguez <mcgrof@suse.com> wrote:
> On Tue, Apr 21, 2015 at 11:39:39PM -0600, Jason Gunthorpe wrote:
>> On Wed, Apr 22, 2015 at 01:39:07AM +0200, Luis R. Rodriguez wrote:
>> > > Mike, do you think the time is right to just remove the iPath driver?
>> >
>> > With PAT now being default the driver effectively won't work
>> > with write-combining on modern kernels. Even if systems are old
>> > they likely had PAT support, when upgrading kernels PAT will work
>> > but write-combing won't on ipath.
>>
>> Sorry, do you mean the driver already doesn't get WC? Or do you mean
>> after some more pending patches are applied?
>
> No, you have to consider the system used and the effects of calls used
> on the driver in light of this table:
>
> ----------------------------------------------------------------------
> MTRR Non-PAT PAT Linux ioremap value Effective memory type
> ----------------------------------------------------------------------
> Non-PAT | PAT
> PAT
> |PCD
> ||PWT
> |||
> WC 000 WB _PAGE_CACHE_MODE_WB WC | WC
> WC 001 WC _PAGE_CACHE_MODE_WC WC* | WC
> WC 010 UC- _PAGE_CACHE_MODE_UC_MINUS WC* | UC
> WC 011 UC _PAGE_CACHE_MODE_UC UC | UC
> ----------------------------------------------------------------------
>
> (*) denotes implementation defined and is discouraged
>
> ioremap_nocache() will use _PAGE_CACHE_MODE_UC_MINUS by default today,
> in the future we want to flip the switch and make _PAGE_CACHE_MODE_UC
> the default. When that flip occurs it will mean ipath cannot get
> write-combining on both non-PAT and PAT systems. Now that is for
> the future, lets review the current situation for ipath.
>
> For PAT capable systems if mtrr_add() is used today on a Linux system on a
> region mapped with ioremap_nocache() that will mean you effectively nullify the
> mtrr_add() effect as the combinatorial effect above yields an effective memory
> type of UC.

Are you sure? I thought that ioremap_nocache currently is UC-, so
mtrr_add + ioremap_nocache gets WC even on PAT systems.

Going forward, when mtrr_add is gone, this will change, of course.

--Andy