Re: Problem with kexec on i386, linux-3.5

From: Khalid Aziz <khalid.aziz@hp.com>
To: "Christian Schaubschläger" <christian.schaubschlaeger@gmx.at>
Cc: kexec@lists.infradead.org, "Eric W. Biederman" <ebiederm@xmission.com>
Subject: Re: Problem with kexec on i386, linux-3.5
Date: Fri, 07 Sep 2012 09:16:33 -0600	[thread overview]
Message-ID: <1347030993.20255.93.camel@lyra> (raw)
In-Reply-To: <502DEB9D.7010308@gmx.at>

On Fri, 2012-08-17 at 08:58 +0200, Christian Schaubschläger wrote:
> >> I bistcted that down to this patch:
> >>
> >> commit b566a22c23327f18ce941ffad0ca907e50a53d41
> >> Author: Khalid Aziz <khalid.aziz@hp.com>
> >> Date:   Fri Apr 27 13:00:33 2012 -0600
> >>
> >>     PCI: disable Bus Master on PCI device shutdown
> >>    
> >>     Disable Bus Master bit on the device in pci_device_shutdown() to ensure PCI
> >>     devices do not continue to DMA data after shutdown.  This can cause memory
> >>     corruption in case of a kexec where the current kernel shuts down and
> >>     transfers control to a new kernel while a PCI device continues to DMA to
> >>     memory that does not belong to it any more in the new kernel.
> >>    
> >>     I have tested this code on two laptops, two workstations and a 16-socket
> >>     server.  kexec worked correctly on all of them.
> >>    
> >>     Signed-off-by: Khalid Aziz <khalid.aziz@hp.com>
> >>     Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
> >>
> >>
> >> Without this patch, int13 works fine here! If anyone needs more
> >> information, just let me know!
> > Which leads to an interesting conundrum.
> >
> > kexec appears to be more reliable for booting another kernel with this
> > patch applied.  This patch does kill the entier use case of making BIOS
> > calls, and I suspect it also does nasty things to alpha bootloaders.
> >
> > My gut feel is that the trampoline code should reenable bus mastering
> > on the devices that lie behind int13, but I don't know how practical
> > that suggestion is in reality.
> 
> One could make this patch optional... e.g. let kexec tell the running kernel whether or not to disable bus mastering before booting the new one (something like 'kexec --disable-bus-mastering -l /boot/new_kernel.bin'). Don't know if this would be practicable...
> 
> Christian
> 

Sorry, I have been on vacation and hadn't been watching mailing list. I
would not want to make this patch optional. It will be difficult for
most end users to tell if they should kexec with this enabled or not. I
would rather fix issues caused by this patch.

I will spend some time understanding the problem you are seeing and get
back to you.

Thanks
-- 
Khalid Aziz <khalid.aziz@hp.com>

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