Re: [PATCH 16/16 v6] PCI: document the new PCI boot parameters
From: Zhao, Yu
Date: Fri Nov 07 2008 - 03:17:22 EST
Greg KH wrote:
On Fri, Nov 07, 2008 at 03:50:34PM +0800, Zhao, Yu wrote:
Greg KH wrote:
On Fri, Nov 07, 2008 at 11:40:21AM +0800, Zhao, Yu wrote:
Greg KH wrote:
On Fri, Nov 07, 2008 at 10:37:55AM +0800, Zhao, Yu wrote:
Greg KH wrote:
This seems like a big problem. How are we going to know to add these
command line options for devices we haven't even seen/known about yet?
How do we know the bus ids aren't going to change between boots (hint,
they are, pci bus ids change all the time...)
We need to be able to do this kind of thing dynamically, not fixed at
boot time, which seems way to early to even know about this, right?
thanks,
greg k-h
Yes, I totally agree. Doing things dynamically is better.
The purpose of these parameters is to rebalance and align resources for
device that has BARs encapsulated in various new capabilities (SR-IOV,
etc.), because most of existing BIOSes don't take care of those BARs.
But how are you going to know what the proper device ids are going to
be before the machine boots? I don't see how these options are ever
going to work properly for a "real" user.
If we do resource rebalance after system is up, do you think there is
any side effect or impact to other subsystem other than PCI (e.g.
MTRR)?
I don't think so.
I haven't had much thinking on the dynamical resource rebalance. If you
have any idea about this, can you please suggest?
Yeah, it's going to be hard :)
We've thought about this in the past, and even Microsoft said it was
going to happen for Vista, but they realized in the end, like we did a
few years previously, that it would require full support of all PCI
drivers as well (if you rebalance stuff that is already bound to a
driver.) So they dropped it.
When would you want to do this kind of rebalancing? Before any PCI
driver is bound to any devices? Or afterwards?
I guess if we want the rebalance dynamic, then we should have it full --
the rebalance would be functional even after the driver is loaded.
But in most cases, there will be problem when we unload driver from a
hard disk controller, etc. We can mount root on a ramdisk and do the
rebalance there, but it's complicated for a real user.
So looks like doing rebalancing before any driver is bound to any device
is also a nice idea, if user can get a shell to do rebalance before
built-in PCI driver grabs device.
That's not going to work, it needs to happen before any PCI device is
bound, which is before init runs.
I don't think it can work either. Then we have to do rebalance after the
driver bounding. But what should we do if we can't unload the driver (hard
disk controller, etc.)?
Well, to do it "correctly" you are going to have to tell the driver to
shut itself down, and reinitialize itself.
Turns out, that doesn't really work for disk and network devices without
dropping the connection (well, network devices should be fine probably).
So you just can't do this, sorry. That's why the BIOS handles all of
these issues in a PCI hotplug system.
How does the hardware people think we are going to handle this in the
OS? It's not something that any operating system can do, is it part of
the IOV PCI spec somewhere?
No, it's not part of the PCI IOV spec.
I just want the IOV (and whole PCI subsystem) have more flexibility on
various BIOSes. So can we reconsider about resource rebalance as boot
option, or should we forget about this idea?
Regards,
Yu
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