linux-kernel - Re: [PATCH 1/3] iommu/amd: Add logic to decode AMD IOMMU event flag

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Date:	Tue, 2 Apr 2013 18:33:18 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	Borislav Petkov <bp@...en8.de>, suravee.suthikulpanit@....com,
	iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] iommu/amd: Add logic to decode AMD IOMMU event flag

On Tue, Apr 02, 2013 at 06:17:57PM +0200, Borislav Petkov wrote:
> On Tue, Apr 02, 2013 at 06:04:00PM +0200, Joerg Roedel wrote:
> > I can certainly write a patch that works around your particular BIOS
> > bug. The problem is that such a fix will most certainly break other
> > systems.
> >
> > Unfortunatly there is no reliable way to fixup the IO-APIC-ID->DEVID
> > mapping at runtime when the BIOS messed it up. The only thing I can do
> > is to check for potential problems and disable the intremap feature
> > then, so that the system will at least boot.
> 
> Yeah, that could work:
> 
> * do not issue message but try to fixup the mapping
> * if it works, fine
> * if it doesn't, then give up and disable intremap.

I can't find out in the driver whether the fix works or not. It will be
noticed later when the x86 code tries to setup the timers and finds out
that they don't work, which causes a kernel panic.

Okay, in theory I could implement a feedback loop between timer-setup
and intremap code and try fixups until it works. But that seems not to
be worth it to work around a buggy BIOS.

What I actually thought about was providing an IVRS-override on the
kernel command line. So that you can specify the IOAPIC_ID->DEVID
mapping there and make it work this way. What do you think?

> And yes, I'm very sceptical about having a WARN_ON and it starts
> screaming on machines all over the place. Good luck explaining to
> people that you actually wanted to prod BIOS vendors to fix their
> monkey-on-crack code but they weren't listening in the first place.

Yeah, that's my fear too. So we leave it better as it is...


	Joerg


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