Re: [Xen-devel] Fwd: [PATCH RFC 3/4] x86/AMD: support further feature masking MSRs

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 07.04.14 at 10:52, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 07/04/14 07:54, Jan Beulich wrote:
>> Andrew otoh, in his attempt to make the feature masking per-VM,
>> will clearly need to take this into consideration, or else he'd be
>> risking runtime crashes.
> 
> That is a good point which I had not anticipated.  The way my fragments
> of per-VM code currently works involve explicitly probing the expected
> set of MSRs at boot, disabling if not found.  In a case like this, the
> fam16 would not get the masks if the microcode started below 0x700010a
> but subsequently got patched higher.
> 
> Catching this case at runtime is hard - if the command line option is
> given, the BIOS is out of date wrt microcode, dom0 boots and applies new
> microcode, it is not generally safe to retroactively apply the boot time
> mask to dom0 which has already started running.

I don't think we should go that far - applying microcode post (Xen-)
boot shouldn't affect past choices. But it of course ought to affect
future ones (i.e. for guests yet to be created).

Jan


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