RE: [git pull] x86 PAT changes

Pallipadi, Venkatesh
2008-04-26T16:52:13Z

>-----Original Message-----
>From: linux-kernel-owner@vger.kernel.org
>[mailto:linux-kernel-owner@vger.kernel.org] On Behalf Of Ingo Molnar
>Sent: Saturday, April 26, 2008 1:56 AM
>To: Linus Torvalds
>Cc: H. Peter Anvin; linux-kernel@vger.kernel.org; Andrew
>Morton; Thomas Gleixner
>Subject: Re: [git pull] x86 PAT changes
>
>
>* Linus Torvalds <torvalds@linux-foundation.org> wrote:
>
>> On Fri, 25 Apr 2008, H. Peter Anvin wrote:
>> >
>> > The problem is that that can create cached/uncached aliases, which
>> > can cause some processors to lock up (especially AMD is known to
>> > have a lot of errata in this area.)
>>
>> Umm.. I don't think you understand. Right now, NONPROMISC_DEVMEM
>> doesn't just disable mmap() on /dev/mem, it disables totally regular
>> reads and writes too. That seems pretty damn excessive.
>>
>> If it was just mmap(), I don't think it would matter much. I don't
>> think we traditionally even supported mmap() on real RAM
>(because the
>> page counting would get confused), and that actually got supported
>> only thanks to VM changes that made it possible.
>>
>> But read/write has always been supported, and shouldn't cause any
>> cached/uncached aliases!
>
>You are right, there should be no architectural need to make PAT
>dependent on nonpromisc-devmem, and thus the patch below
>should be safe.
>

Agreed that NONPROMISC_DEVMEM is not really needed for read/write. But,
we will still need it for /dev/mem.

The problem with /dev/mem maps of RAM is situation like this:
1) drivers does vmalloc(), followed by set_memory_uc.
2) User does a /dev/mem map of that vmalloced physical address. User
will get a UC mapping for /dev/mem.
3) driver changes the memory to set_memory_wb and frees the memory.
4) user mapping for this address is still UC which will lead to
aliasing.

Read/write is ok, as they will just use __va for RAM to access and that
will always be consistent.

Thanks,
Venki