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Date: Thu, 09 Feb 2012 18:20:38 +0100
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Subject: Re: [Qemu-devel] [PATCH 3/6] kvmvapic: Introduce TPR access
 optimization for Windows guests
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To: Jan Kiszka <jan.kiszka@web.de>
Cc: Anthony Liguori <aliguori@us.ibm.com>, kvm@vger.kernel.org, Gleb Natapov <gleb@redhat.com>, Marcelo Tosatti <mtosatti@redhat.com>, qemu-devel <qemu-devel@nongnu.org>, Avi Kivity <avi@redhat.com>

On 02/09/2012 05:32 PM, Jan Kiszka wrote:
> >  I mean just check kpcr.self.
>
> Yes, clear, but that means that Windows must have initialized FS.base to
> point to the KPCR also in UP mode. Is that really the case? E.g. when
> ACPI is off?! I wonder if that explains the reported bug of qemu-kvm
> with -no-acpi and in-kernel irqchip...

Yes, it does.  It's used by some fast-path kernel APIs, and indeed the 
canonical way to find the KPCR base from ring 0 is to look at FS:[1Ch].

Similarly in userspace you can find the thread information block at 
FS:[sizeof(void*)*6], and FS:[1Ch] is something else.  But your code 
cannot be reached from userspace, so that's always fine.

Paolo