linux-kernel - Re: [RFCv3][PATCH 2/3] fix kvm's use of _

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Message-ID: <50F5A215.5020708@redhat.com>
Date:	Tue, 15 Jan 2013 13:38:13 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
CC:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Gleb Natapov <gleb@...hat.com>, Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [RFCv3][PATCH 2/3] fix kvm's use of __pa() on percpu areas

On 01/09/2013 01:59 PM, Dave Hansen wrote:
> In short, it is illegal to call __pa() on an address holding
> a percpu variable.  The times when this actually matters are
> pretty obscure (certain 32-bit NUMA systems), but it _does_
> happen.  It is important to keep KVM guests working on these
> systems because the real hardware is getting harder and
> harder to find.
>
> This bug manifested first by me seeing a plain hang at boot
> after this message:
>
> 	CPU 0 irqstacks, hard=f3018000 soft=f301a000
>
> or, sometimes, it would actually make it out to the console:
>
> [    0.000000] BUG: unable to handle kernel paging request at ffffffff
>
> I eventually traced it down to the KVM async pagefault code.
> This can be worked around by disabling that code either at
> compile-time, or on the kernel command-line.

Acked-by: Rik van Riel <riel@...hat.com>


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