netdev - Re: Expose ltr/obff interface by sysfs

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Date:	Fri, 6 Apr 2012 08:26:49 -0700
From:	Jesse Barnes <jbarnes@...tuousgeek.org>
To:	"Hao, Xudong" <xudong.hao@...el.com>
Cc:	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"e1000-devel@...ts.sourceforge.net" 
	<e1000-devel@...ts.sourceforge.net>
Subject: Re: Expose ltr/obff interface by sysfs

On Fri, 6 Apr 2012 02:43:59 +0000
"Hao, Xudong" <xudong.hao@...el.com> wrote:

> Hi, 
> 
> I'm working on virtualization Xen/KVM. I saw there are ltr/obff enabling/disabling function in pci.c, but no called till now. I want to know if anybody(driver developer) are working for using it? Can driver change the LTR latency value dynamically?

I believe the value is writable, but I'd expect some devices to
misbehave if the value were programmed too high.  Performance would
also suffer if the value were set too high, at least for IOPS sensitive
devices.

> /*
> LTR(Latency tolerance reporting) allows devices to send messages to the root complex indicating their latency tolerance for snooped & unsnooped memory transactions.
> OBFF (optimized buffer flush/fill), where supported, can help improve energy efficiency by giving devices information about when interrupts and other activity will have a reduced power impact.
> */
> 
> One way to control ltr/obff is used by driver, however, I'm considering that in virtualization, how guest OS driver control them. I have an idea that expose an inode interface by sysfs, like "reset" inode implemented in pci-sysfs.c, so that system user/administrator can enable/disable ltr/obff or set latency value on userspace, but not limited on driver. Comments?

Given how device specific these extensions are, I'd expect you'd need
to know about each specific device anyway, which is why I think the
control belongs in the driver.  I don't see why you'd need to
enable/disable/change these functions when assigning a device from one
guest to another...

-- 
Jesse Barnes, Intel Open Source Technology Center

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