Re: [Xen-devel] [DESIGN] Feature Levelling improvements

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 16.06.15 at 12:50, <andrew.cooper3@xxxxxxxxxx> wrote:
> How XenServer currently does levelling
> ======================================
> 
> The _Heterogeneous Pool Levelling_ support in XenServer appears to
> predate the
> libxc CPUID policy API, so does not currently use it.  The toolstack has a
> table of CPU model numbers identifying whether levelling is supported.  It
> then uses native `CPUID` instructions to look at the first four feature
> masks,
> and identifies the subset of features across the pool.
> `cpuid_mask_{,extd_}{ecx,edx}` is then set on Xen's command line for
> each host
> in the pool, and all hosts rebooted.
> 
> This has several limitations:
> 
> * Xen and dom0 have a reduced feature set despite not needing to migrate

I don't think Xen is affected by this, as it reads the CPUID bits
before setting the masks (there are a few cpuid() invocations
in "random" code, but I don't think these access maskable ones).

> Notes and observations
> ======================
> 
> Experimentally, the masking MSRs can be context switched.  There is no
> need to
> force all PV guests to the same level, and no need to prevent dom0 or
> Xen from
> using certain features.  Context switching the masking MSRs will however
> incur
> an overhead, and should be avoided where possible.
> 
> The toolstack needs to know how much control Xen has over VM features. 
> In the
> case that there are insufficient masking MSRs, and no faulting support is
> present, a PV VM can still potentially be made safe to migrate by explicitly
> disabling features on the kernel command line.

That wouldn't help with user mode code, would it?

> VCPU context switch
> -------------------
> 
> Xen shall be updated to lazily context switch all available masking
> MSRs.  It
> is noted that this shall incur a performance overhead if restricted
> featuresets are assigned to PV guests, and _CPUID Faulting_ is not
> available.
> 
> It shall be the responsibility of the host administrator to avoid creating
> such a scenario, if the performance overhead is a concern.

Not sure how feasible this is: Even if you run all PV guests at equal
feature levels, context switching between PV and non-PV guests
would still incur overhead (unless you mean to run HVM/PVH ones
with whatever masking is currently in place). Plus this still wouldn't
deal with masks in place when Xen itself wants to look at any of the
maskable ones, unless you intend to audit code to make sure no
such uses exist (which - as per above - I suppose/hope to be the
case).

> Future work
> ===========
> 
> The above is a minimum quantity of work to support feature levelling, but
> further problems exist.  They are acknowledged as being issues, but are
> not in
> scope for fixing as part of feature levelling.
> 
> * Xen has no notion of per-cpu and per-package data in the cpuid policy.  In
>   particular, this causes issues for VMs attempting to detect topology,
> which
>   find inconsistent/incorrect cache information.
> 
> * In the case that `domain_cpuid()` can't locate a leaf in the topology, it
>   will fall back to issuing a plain `CPUID` instruction.  This breaks VM
>   encapsulation, as a VM which has migrated can observe differences which
>   should be hidden.

I think this is actually something that (a) needs addressing not too
far in the future and (b) reminds me that I didn't see any talk here
regarding black vs white listing of features not explicitly known to
Xen or the tool stack.

Jan


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