linux-kernel - Re: [PATCH for-2.6.35] virtio-pci: disable msi at startup

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Date:	Wed, 23 Jun 2010 08:51:53 -0700
From:	Jesse Barnes <jbarnes@...tuousgeek.org>
To:	Avi Kivity <avi@...hat.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	virtualization@...ts.linux-foundation.org,
	Anthony Liguori <aliguori@...ibm.com>,
	linux-pci@...r.kernel.org, Matt Carlson <mcarlson@...adcom.com>,
	linux-kernel@...r.kernel.org, "Rafael J. Wysocki" <rjw@...k.pl>,
	Kenji Kaneshige <kaneshige.kenji@...fujitsu.com>,
	Tejun Heo <tj@...nel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Bjorn Helgaas <bjorn.helgaas@...com>
Subject: Re: [PATCH for-2.6.35] virtio-pci: disable msi at startup

On Wed, 23 Jun 2010 18:35:55 +0300
Avi Kivity <avi@...hat.com> wrote:

> On 06/23/2010 06:26 PM, Michael S. Tsirkin wrote:
> >
> >
> >>>
> >>>        
> >>>>>> Shouldn't a reset be equivalent to power cycling?
> >>>>>>
> >>>>>>
> >>>>>>              
> >>>>> If we did this, driver would need to restore registers
> >>>>> such as BAR etc.
> >>>>>
> >>>>>
> >>>>>            
> >>>> We could save/restore the registers we care about.
> >>>>
> >>>>          
> >>> It seems easier to clear registers we care about.
> >>>        
> >> We know the registers we care about, we don't know the ones we don't.
> >>      
> > If/when we use more registers, we can update driver to clear them on start.
> >    
> 
> The kdump kernel may not load drivers for those extra devices.

FLR or another type of reset also has the nice property of bringing the
device into a known state.  kexec/kdump has always been vulnerable to
having devices in partial states when the new kernel loads; would be
good to make it more robust.

> 
> >> I'm talking about FLRing all cards, not just those you want to use.
> >>      
> > reset using FLR/PM is complex because of the need to save/restore
> > config space. Doing this on a crashing kernel sounds scary.
> >    
> 
> Well, you only need to save/restore for the devices you use.  The rest 
> you reset and forget.
> 
> I don't really see why copying some config space is crazy.

We could push any needed save/restore of core settings and regs into
the PCI core like we do for PM.  That would save a bunch of driver
trouble...

-- 
Jesse Barnes, Intel Open Source Technology Center
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