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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

Can people still play with the CPUID policy if they really need to
disable it?  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.

~Andrew

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