Re: [patch] kvm: make cr3 loading more robust

[Ingo Molnar <mingo-X9Un+BFzKDI@public.gmane.org>]

* Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org> wrote:

> Note that the corner cases will never be 100% emulatable.  For 
> example, you can set cr3 to point at your IDE DMA mmio space or 
> something like that.  It's quite all right to kill the guest quietly 
> at that point, as no real-life guest will do that.

yes. Or to map the lapic to the IDT ;-) (as yours truly has tried it 
years ago)

that's why my suggestion is to just kill the guest. Loading such a cr3 
is a serious bug that might be hard to debug in the guest. I had to 
debug at least one such bug in Linux before (years ago, in the lazy TLB 
switching code) and it was a royal PITA to track down. Having a 
hypervisor that points any cr3 load error out /before/ the effects of 
the error propagate further is a bonus, not an incompatibility. The CPU 
does not implement this not because the semantics is important, but i 
suspect mostly because it doesnt really know the boundaries and type of 
RAM.

> The kvm goals do not include cycle accurate emulation. [...]

yes. That's why i'm suggesting to kill the VM in such a scenario. A cr3 
value is only valid if it points to real RAM.

	Ingo

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