[libvirt] [PATCH v5 7/9] tests: check CPU features handling in libxl driver

Mon Mar 19 15:14:20 UTC 2018

On Mon, Mar 19, 2018 at 02:11:02PM +0000, Daniel P. Berrangé wrote:
> On Fri, Mar 16, 2018 at 08:54:42PM +0100, Marek Marczykowski-Górecki wrote:
> > On Fri, Mar 16, 2018 at 05:44:28PM +0000, Daniel P. Berrangé wrote:
> > > Since Xen lets you specify raw  "cpuid" register values here, surely
> > > this is flexible enough to allow us to support the mode=custom CPU
> > > models ?
> > > 
> > > We would just need to make sure every bit poisition used either
> > > 0 or 1, and not 'x', so that we are fully overriding whatever
> > > defaults are presented by the hypervisor "host" CPU model. Or is
> > > life more complicated than that ?
> > 
> > This is what v1 of this series had... See discussion there, especially
> > message from Jiri:
> > https://www.redhat.com/archives/libvir-list/2017-June/msg01304.html
> > 
> > And this, from... you:
> > https://www.redhat.com/archives/libvir-list/2017-June/msg01308.html
> 
> Ok, yeah, I remember these discussions now.  What I didn't realize at the
> time though, was that the revised patches would end up silently changing
> any XML that says mode="custom" + model="Foobar", into mode="host-passthrough"
> 
> The core problem we faced in QEMU was that if you take a base named CPU
> model and then add/remove 20+ features, some guest OS can get confused.
> This is because the combination of features may be unusual, or the
> level & xlevel values reported alongside CPUID are unsual for the given
> set of features enabled.  The Xen config doesn't provide a way to configure
> the level & xlevel values for the CPU, so will potentially hit this same
> problem. This is *already* possible though, even with host-passthrough,
> since the user can ask for host-passthrough and also disable a whole
> bunch of features.
> 
> IOW, Jiri is right, but I don't think that neccesarily implies we should
> not implement support for mode="custom". I see we have 3 options here
> currently
> 
>  1. Support mode="custom", using the cpuid config option in libxl
>  2. Raise an error when starting a guest with mode="custom"
>  3. Translate mode="custom" into mode="host-passthrough" and ignore
>     requested named model
> 
> So currently situation is that users of libxl libvirt driver may have
> deployed guests using arbitrary <cpu> model setup, and that would have
> been ignored, giving that a host-passthrough setup.
> 
> If we choose option (1), users will have changed semantics for their
> existing guests, as instead of getting full host-passthrough, they'll
> get a CPU that actually matches the mode=custom their XML has beeen
> requesting all along.
> 
> If we choose option (2), we'll break existing deployed guests with
> mode=custom.
> 
> If we choose option (3), users will have preserved semantics for their
> existing guests, but we'll never be honouring the actual XML config
> the user requested. If we did ever want to properly support mode=custom
> in future we're still going doom existing users.

There is a variant of this option: do not translate mode="custom" into
mode="host-passthrough" explicitly (keep it mode="custom" in the
config), but ignore both model and features in mode="custom" - as it is
currently.
In other words - add CPUID support only to mode="host-passthrough" and
keep other modes untouched.
Implementing proper mode=custom support (either now or later) will
change guest view of the system anyway, it doesn't matter when we do it.
And I'm leaning to doing it later.

> Ideally we would have choosen option (2) from day 1, but that ship
> has sailed.
> 
> I really dislike, even hate, option (3) because it is explicitly
> ignoring a valid XML configuration request and turning it into
> something completely different.

Well, libvirt does it all the time - by ignoring not supported domain
config elements instead of raising some error. For example most
<feature policy=...> are in this category, regardless of the mode, until
this patch series. Same for <cputune>, <memtune>, and other <*tune>,
various hypervisor features (libxl supports apic, pae, and acpi, others
are silently ignored). I can go on with the list...
And I guess it isn't only libxl driver.

I'm not saying it's fundamentally wrong - it make it easier to write
universal domain XML configuration that will work on any libvirt
version. One just need to check if given feature is supported by
particular libvirt version.
What I propose here is to keep this behavior for mode=custom.

I don't want to implement both mode=custom and mode=host-passthrough
in this patch series, mostly for non-technical reasons. It already took
very long time and I'd like to finally have at least some of it in. And
also it looks like mode=custom will be mostly separate code (maybe even
using old xend syntax, to have control over all cpuid bits, not only
those listed in libxl_cpuid.c).

> So despite the problems / limitations of mode=custom that were previously
> raised, I none the less think we should implement mode=custom for the
> libxl driver. Then document that its usage is discouraged - for the same
> reason that we discourage users from arbitrarily blanking out features
> for mode=host-passthrough.
> 
> Perhaps in future, we could make mode=custom work more reliably if the
> libxl driver provided a way to set the level & xlevel values.

What exactly do you mean by level & xlevel values?
libxl cpuid syntax supports maxleaf and maxhvleaf - is it the same?
See here for details:
http://xenbits.xen.org/docs/unstable/man/xl.cfg.5.html#cpuid-LIBXL_STRING-or-cpuid-XEND_STRING-XEND_STRING

> > This is not only about 'x'. But also about setting '1' where hardware
> > does not really support given feature. This will also result in "broken"
> > CPU.
> 
> If we host hardware does not support a given feature bit, then the code
> has to make a decision based on the <feature policy=...> setting. ie
> "force" would present the feature to the guest, regardless of whether
> the host supports ir, while "require" would refuse to start the guest,
> and "optional" would silently disable the feature.

Hmm, but <feature policy=...> applies only to bits you explicitly
specify, not all those implied by given model, right? This means that
innocent model setting may also result in weird configuration. This is
nothing libxl specific, just about mode=custom on hardware assisted
virtualization (as opposed to pure software one, that could provide some
features regardless of host support).

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?
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