Re: [PATCH] X86: MM: Add PAT Type write-through in combination with mtrr

[Ingo Molnar <mingo@kernel.org>]

* Andreas Werner <wernerandy@gmx.de> wrote:

> On Mon, Oct 28, 2013 at 11:31:32AM +0100, Ingo Molnar wrote:
> > 
> > * Borislav Petkov <bp@alien8.de> wrote:
> > 
> > > On Mon, Oct 28, 2013 at 11:17:49AM +0100, Ingo Molnar wrote:
> > >
> > > > And regular write-back cacheable isn't sufficient because the 
> > > > CPU could do things like prefetch your range automatically?
> > > 
> > > Yeah, he's doing a CLFLUSH anyway which basically makes it a 
> > > write-through...
> > 
> > The CLFLUSH is done afterwards (making it a use-once thing), so WB 
> > might still be faster and would avoid the PAT headache ...
> > 
> > Thanks,
> > 
> > 	Ingo
> What i do right now is:
> 1. clflush the data range to read from my mmio device
> 2. read the data.
> On PCIe Tracer i see the pcie  bursts.
> 
> If i mark the region WB and call clflush my system will crash without
> any message, it just stop working.

Yeah, I was wondering whether it's valid at all to mark IO memory as 
cacheable - with the lack of MESI transactions and all that ...

So it's apparently not valid and we've got to live with WT as the 
'best' caching/bursting method for reads.

Thanks,

	Ingo