[Qemu-devel] Re: [PATCH 2/2] introduce -cpu host target

[Andre Przywara]

Avi Kivity wrote:
> On 06/23/2009 12:47 AM, Andre Przywara wrote:
> > Should we ignore unhandled MSRs like QEMU or Xen do?
> >    
>
> Ignoring unhandled msrs is dangerous.  If a write has some effect the 
> guest depends on, and we're not emulating that effect, the guest will 
> fail.  Similarly if you don't know what a register mean, who knows what 
> returning zero for a read will do.
I agree - from an academic POV.
But if the pragmatic approach simply enables many guests to run, then 
it's at least worth considering it.
And with the current approach the guest fails, too (due to the injected 
#GP).
If I only look at AMD's list of MSRs (not to speak of the internal list 
;-), there will be a lot of work to emulate them. Even worse, most of 
them cannot be properly emulated (like disable Lock prefix).

But nevertheless I would like to continue the "patch-on-demand" path by 
catching those MSRs that in-the-wild OSes really touch and handle them 
appropriately. Hopefully that will cover most of the MSRs.
Maybe we could consider an (module? QEMU cmdline?) option to ignore 
unknown MSRs.

Regards,
Andre.

--
Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany
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