Re: x2APIC MSR range (XSA-108 follow-up)

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 16/10/14 08:57, Matt Wilson wrote:
> On Tue, Oct 14, 2014 at 06:23:52AM +0000, Zhang, Yang Z wrote:
>> Wu, Feng wrote on 2014-10-14:
>>> The SDM 10.12.1.2 says:
>>>
>>> " Addresses in the range 800H¨CBFFH that are not listed in Table 10-6
>>> (including 80EH and 831H) are reserved.
>>> Executions of RDMSR and WRMSR that attempt to access such addresses
>>> cause general-protection exceptions. "
>>>
>>> Table 10-6. Local APIC Register Address Map Supported by x2APIC
>>>
>>> Why should we virtualize those reserved MSRs for guests?
>> IIUC the question should be why those undocumented MSRs exist on
>> real hardware and what do they do? Will guest access to them via
>> check CPU model? If yes, how can we virtualize them correctly?
> I suspect that Intel knows what they are and what they do. I imagine
> that both are CPU model specific.
>
>> I don't have the answer right now but I will forward the question to
>> hardware guy for more help.
> Since these MSRs are not part of the SDM or any public platform
> documentation as far as I can tell, I imagine that they are for
> BIOS-level functionality that does not have meaning in a virtual
> environment today and presents no OS compatibility problem if we
> choose to #GP all access to them.
>
> Short of some reply from Intel saying "No! That will break some OS" I
> think that we should make the x2APIC MSR code handle 0x800...0xbff and
> #GP any access that is above the emulated area.

I have a patch like this currently getting a kicking in our dedicated
XSA-108 test set.  I will report back with how it does.

~Andrew