Re: [Qemu-devel] [PATCH 3/6] kvmvapic: Introduce TPR access optimization for Windows guests

[Paolo Bonzini]

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