Re: [Xen-devel] [PATCH 0/9] xen/amd: Support for guest MSR_VIRT_SPEC_CTRL support

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 04.12.18 at 12:26, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 04/12/2018 09:45, Jan Beulich wrote:
>> Nor can I see how hiding these MSRs from guests would improve
>> the situation in this regard: Guests may still draw unwanted
>> conclusions from not being able to read these MSRs, or reading
>> all zeros.
> 
> I can't help but feel that the observations you've made answer the
> question very succinctly.
> 
> Of course we can't prevent the guest drawing conclusions from the
> absense/presence of the information.  What we can (and must) ensure is
> that the information that is available (i.e. a #GP fault) does not have
> any details which are specific to the processor that the VM happened to
> boot on.

But that's the issue: Even #GP on such an MSR access convey
information. An OS may legitimately assume
- no #GP based on the family/model/stepping values
- old hardware if #GP is observed upon reading (which in turn
  may mean it works in a sub-optimal way)
- brokenness if no #GP but an all zero value, but if the BKGD
  documents certain bits to be set (perhaps by the BIOS)
- whatever else

What I'm trying to express is: We simply can't get this right
unless we _fully_ emulate family/model/stepping specific
behavior (according to the values seen by the guest), with
or without migration.

Jan



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