linux-kernel - Re: pgprot_writecombine() and PATs on x86

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Message-ID: <adaslaowbgn.fsf@cisco.com>
Date:	Wed, 25 Apr 2007 11:33:28 -0700
From:	Roland Dreier <rdreier@...co.com>
To:	Andi Kleen <ak@...e.de>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org, mst@...lanox.co.il,
	jackm@...lanox.co.il
Subject: Re: pgprot_writecombine() and PATs on x86

 > > Where do your patches to add an implementation of
 > > pgprot_writecombine() using PATs on x86 stand?  
 > 
 > It's on my todo list.

Great.  Let me know if there's anything I can do to help.

 > When it's PCI space you can likely just use MTRRs. PAT is mostly useful
 > for applications that do IO with random memory pages

Actually MTRRs seem to be inadequate for a number of reasons.  For
example I have a system where /proc/mtrr looks like:

	$ cat /proc/mtrr 
	reg00: base=0x00000000 (   0MB), size=8192MB: write-back, count=1
	reg01: base=0x200000000 (8192MB), size= 512MB: write-back, count=1
	reg02: base=0x220000000 (8704MB), size= 256MB: write-back, count=1
	reg03: base=0xd0000000 (3328MB), size= 256MB: uncachable, count=1
	reg04: base=0xe0000000 (3584MB), size= 512MB: uncachable, count=1

And I want to map the second half of the second BAR of this device
with write-combining:

	0d:00.0 InfiniBand: Mellanox Technologies Unknown device 634a (rev a0)
		Subsystem: Mellanox Technologies Unknown device 634a
		Flags: bus master, fast devsel, latency 0, IRQ 16
		Memory at fc400000 (64-bit, non-prefetchable) [size=1M]
		Memory at d8000000 (64-bit, prefetchable) [size=8M]
		Memory at fc3fe000 (64-bit, non-prefetchable) [size=8K]
		Capabilities: <access denied>

So it's not clear that there will be enough MTRRs to handle
everything, or that even if there are enough, that there's a safe way
to update the MTRRs to get from the boot-up config to the one we want.
In this case I guess there is a way but it uses all 8 MTRRs, so adding
a device that also wants write combining won't work.

And definitely trying to set up the MTRRs automatically is going to to
be very fragile.  So I think having pgprot_writecombine() implemented
with PATs is really the only sane thing even for this PCI space.

 - R.
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