On Thursday, April 17, 2008 6:14 am Pallipadi, Venkatesh wrote:
quoted text
> >Jesse Barnes suggested using the fact that the BAR was prefetchable to
> >guess that this region should not be mapped UC. I don't know of any
> >other information available at this API that would help make a better
> >choice; there are no ioctls on /sys files that we could use to
> >manipulate the mapping.
>
> Yes. But, the API being there for some time may mean that there is some
> user who always wants UC behavior with or without PREFETCHABLE flag.
> With PAT changes we thought of ioctl and then settled on new interface.

Yeah, we can't really know.  There *probably* aren't any users here that would 
break, but you never know, and it's best not to change the behavior of an 
existing API...

quoted text
> >Yeah, we'll do that when it becomes available. Would it make sense in
> >the pre-PAT world to use Jesse's guess?
>
> Yes. Easiest way to do that will be to use UC_MINUS (Just set PCD bit
> and not PWT bit) until PAT changes.
> Ioremap() uses UC_MINUS, it should not have any side effects to other
> users who expect this mapping to be UC without any MTRR setting.

We could probably do that unconditionally though, rather than dependent on the 
prefetchable bit like my patch does.

quoted text
> >In any case, we'll continue to use the fact that mprotect is
> >also broken
> >to get our WC mapping working (using mprotect PROT_NONE followed by
> >mprotect PROT_READ|PROT_WRITE causes the CD and WT bits to get
> >cleared).
> >We're fortunate in this case that we've found a bug to exploit that
> >gives us the desired behaviour.
>
> Noo.. Now we have one more thing in PAT to do list :-(. To go and plug
> those mprotect APIs to prevent users from doing things like this.

I think it's just a long standing bug...  mprotect really shouldn't be 
clobbering PTE bits regardless of PAT (though with PAT the bug becomes more 
serious).

Jesse
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