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

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Message-ID: <20131028105101.GC6274@gmail.com>
Date:	Mon, 28 Oct 2013 11:51:01 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Andreas Werner <wernerandy@....de>
Cc:	Borislav Petkov <bp@...en8.de>, tglx@...utronix.de,
	mingo@...hat.com, hpa@...or.com, x86@...nel.org,
	dave@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] X86: MM: Add PAT Type write-through in combination with
 mtrr


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

> On Mon, Oct 28, 2013 at 11:31:32AM +0100, Ingo Molnar wrote:
> > 
> > * Borislav Petkov <bp@...en8.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
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