Re: [Xen-devel] [libvirt] Question about hypervisor <features> that are not tristate

[Jim Fehlig <jfehlig@xxxxxxxx>]

On 01/11/2017 11:00 AM, Andrew Cooper wrote:
> On 11/01/17 17:49, Jim Fehlig wrote:
> > On 01/06/2017 05:31 PM, Jim Fehlig wrote:
> >
> > Adding xen-devel for a question below...
> >
> > > Happy new year!
> > >
> > > Nearly a year ago I reported an issue with the <hap> hypervisor
> > > feature on Xen
> > > [1] and am now seeing a similar issue with the <pae> feature. Setting
> > > the
> > > default value of pae changed between xend and libxl. When not
> > > specified, xend
> > > would enable pae for HVM domains. Clients such as xm and the old
> > > libvirt driver
> > > did not have to explicitly enable it. In libxl, the pae field within
> > > libxl_domain_build_info is initialized to 0. Clients must enable pae,
> > > and indeed
> > > xl will do so if pae=1 is not specified in the xl.cfg.
> > >
> > > The xend behavior prevents libvirt from disabling pae, whereas the
> > > libxl behvior
> > > causes a guest ABI change (config that worked with libvirt+xend
> > > doesn't with
> > > libvirt+libxl). The libxl behavior also forces management software (e.g.
> > > OpenStack nova) to add <pae> where it wasn't needed before.
> > >
> > > To solve this problem for <hap>, it was changed it to a tristate [2],
> > > allowing
> > > it to be turned off with explicit <hap state='off'/>, and on if not
> > > specified or
> > > <hap/> or <hap state='on'/>. Should <pae> (and the remaining
> > > hypervisor features
> > > that are not tristate) be converted to tristate similar to <hap>?
> > > Alternatively,
> > > I could simply set pae=1 for all HVM domains in the libxl driver.
> > > Like the old
> > > libvirt+xend behavior it couldn't be turned off, but I don't think
> > > there is a
> > > practical use-case to do so. At least no one has complained over all
> > > the years
> > > of libvirt+xend use.
> > Xen folks, what is your opinion of always enabling pae for HVM domains
> > in the libvirt libxl driver? Is there a need these days to disable it?
> >
> > Jan had mentioned that some old, buggy guest OS's (Win 9x) might need
> > it disabled, and perhaps some cases where it may be desirable to
> > suppress a guest OS entering 64-bit mode. But in practice do you think
> > it is necessary to expose this knob to users?
> ISTR the main use of this knob being to cause 32bit versions of windows
> to avoid using PAE paging, making them more efficient to shadow.
>
> Now that 64bit and EPT/NPT are fairly ubiquitous, there should be no
> reason to turn it off.
Okay, thanks.

> Can people still play with the CPUID policy if they really need to
> disable it?
ATM, not through libvirt. We still have some work to do in this area.

> It is unfortunate that this option was ever exposed via a
> non-CPUID mechanism, but I am trying to clean this up at the hypervisor
> interface to ensure that the *only* way to alter details like this is
> via the appropriate interface.
And we'll need to make use of this interface in libvirt.

Regards,
Jim


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