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

Ingo Molnar
2013-10-28T10:51:07Z

* 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