Re: [Xen-devel] [PATCH 3/4 V3] X86: MPX IA32_BNDCFGS msr handle

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Thu, 2013-11-28 at 12:14 +0100, Tim Deegan wrote:
> At 11:12 +0000 on 28 Nov (1385633520), Liu, Jinsong wrote:
> > Tim Deegan wrote:
> > > At 03:17 +0000 on 28 Nov (1385605079), Liu, Jinsong wrote:
> > >> Andrew Cooper wrote:
> > >>> On 27/11/13 15:02, Liu, Jinsong wrote:
> > >>>> Andrew Cooper wrote:
> > >>>>> It is very common to have pools of servers made of different
> > >>>>> generations of CPU.  E.g. Ivy Bridge and Haswell.  To safely
> > >>>>> migrate a VM, the feature set the VM can see must be the common
> > >>>>> subset of the two. 
> > >>>>> 
> > >>>>> ~Andrew
> > >>>> Yes -- but that's not a reason to prevent MPX feature (or, any new
> > >>>> features) -- otherwise you have to prevent any new features.
> > >>>> The right place to control cpuid policy of a pool is at higher
> > >>>> level, where it has full information of the pool machines and so
> > >>>> it's right place to make decision what cpuid feature set would be
> > >>>> proper for the specific pool. 
> > >>>> 
> > >>> That is exactly a reason to prevent MPX.
> > >>> 
> > >>> If the domain cpuid policy (which is set by the toolstack) states
> > >>> that MPX should be disabled, then MPX must be hidden from the HVM
> > >>> guest, even if the hardware supports MPX.
> > >> 
> > >> No. That's _not_ a reason to prevent MPX -- toolstack still has the
> > >> right to disable MPX, no matter h/w support MPX or not. Refer
> > >> xc_cpuid_set().
> > > 
> > > There seems to be a lot of confusion here.  As far as I can tell, the
> > > only sensible mechanism is:
> > > 
> > > - If the hardware doesn't support MPX, mask it in guest CPUID.
> > > - If the domain cpuid policy masks the MPX feature, disable it.
> > > - Otherwise, enable it, and advertise it in guest CPUID.
> > > 
> > > In any case, the CPUID fields seen by the guest _must_ match whether
> > > the feature is available.
> > > 
> > > Tim.
> > 
> > Do you mean xc_cpuid_set() is some confusion? Yes, it's some buggy that 
> > need got fix at tools side.
> > 
> > I take it here as an example just indicate 'toolstack has the right to 
> > disable/mask hardware feature, if it want to do so per domain cpuid policy'.
> > 
> 
> In that case, I think you and Andrew are agreeing with each other. :)
> The important detail is that if the toolstack has disabled the feature
> using the CPUID policy, the hypervisor should not expose the feature
> to the guest.

Do you mean expose as in "present in cpuid" or do you mean expose as in
"the instructions/features work (even if disabled in cpuid)"?

Ian.


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