linux-kernel - Re: [PATCH] Align PCI memory regions to page size (4K)

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Message-ID: <20071114062157.GA30203@colo.lackof.org>
Date:	Tue, 13 Nov 2007 23:21:57 -0700
From:	Grant Grundler <grundler@...isc-linux.org>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	David Miller <davem@...emloft.net>, gregkh@...e.de,
	barak@...cleus.com, linux-kernel@...r.kernel.org,
	linux-pci@...ey.karlin.mff.cuni.cz, guy@...cleus.com
Subject: Re: [PATCH] Align PCI memory regions to page size (4K) - Fix

On Wed, Nov 14, 2007 at 08:17:33AM +1100, Benjamin Herrenschmidt wrote:
...
> Though he's trying to fix a real issue, his patch is not the right
> approach imho.
> 
> A better approach would be to have a mechanism to be triggered by the
> hypervisor administration tools that will attempt to reassign -that-
> specific device if it happens to share pages with another.
> 
> The remapping would thus only happen for that single device, after it's
> been put in control of the hypervisor (no host driver is bound, maybe
> just an HV specific "bridging" driver is), and only when the action of
> assigning it to the partition is performed, so that if the machine
> crashes as a result, at least you know why :-)
> 
> So something like your hypervisor binds a special driver to a device
> that is to be reflected to a partition, at which point we are sure no
> other driver is using it, then that driver can call something in the pci
> layer that attempts to re-assign the device resources to keep it in a
> separate page.

We already have that "something": pci_enable_device().
The guest OS "Arch" code can then do the reassignment.
See "3.1 Enable the PCI device" in Documentation/pci.txt.


> A patch implementing such a helper, and maybe reserving the rest of the
> MMIO page via some dummy resource to make sure nobody else gets in, that
> would make more sense.

The Hypervisor could be responsible for making the right devices
visible to the appropriate partitions/guests by intercepting the
PCI bus walk and/or hotplug support. I don't think you
should need any dummy resource/drivers in the guest OS.

hth,
grant
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