Re: [Xen-devel] [PATCH] libxc: Expose the pdpe1gb cpuid flag to guest

[Tim Deegan <tim@xxxxxxx>]

At 16:40 +0000 on 17 Nov (1416238835), Andrew Cooper wrote:
> On 17/11/14 16:30, Tim Deegan wrote:
> > At 16:24 +0000 on 17 Nov (1416237888), Jan Beulich wrote:
> >>>>> On 17.11.14 at 16:39, <Ian.Jackson@xxxxxxxxxxxxx> wrote:
> >>> Liang Li writes ("[PATCH] libxc: Expose the pdpe1gb cpuid flag to guest"):
> >>>> If hardware support the pdpe1gb flag, expose it to guest by default.
> >>>> Users don't have to use a 'cpuid= ' option in config file to turn
> >>>> it on.
> >>> I don't understand what this flag does.  I guess from the name it
> >>> turns on 1G pages.  I guess these are supported ?
> >>>
> >>> I would like to see comment from an x86 hypervisor maintainer.
> >> Yes, we support 1Gb pages. The purpose of the patch is to not
> >> unconditionally hide the flag from guests. (I had commented on
> >> v1, but sadly this one wasn't tagged as v2, nor was I included on
> >> the Cc list, hence I didn't spot the new version.)
> >>
> >> The one thing I'm not certain about is shadow mode: Only 2Mb
> >> pages may be supported there. Tim?
> > Indeed, only 2MiB pages are supported in shadow mode.  See, e.g.
> > guest_supports_1G_superpages()->hvm_pse1gb_supported()->paging_mode_hap()
> 
> This is yet another case which proves that libxc is the wrong place to
> be choosing the cpuid flags exposed to a domain.
> 
> Furthermore, guest_supports_1G_superpages() is insufficient as it only
> checks whether the host is capable of providing 1G superpages, not
> whether the guest has been permitted to use it.
> 
> This causes a problem when migrating between hap-capable and
> hap-incapable systems.

There's no notion of 'permitted' to use 1G pages, AFAICS, much like
other CPU features.  But of course a _well-behaved_ guest that has
been told in cpuid not to use 1G superpages will have no problems. :)

> I do hope to fix all of this with my planned changes to the cpuid
> infrastructure.

Good luck!

Tim.

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