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

[Tim Deegan <tim@xxxxxxx>]

At 17:25 +0000 on 17 Nov (1416241517), Andrew Cooper wrote:
> On 17/11/14 17:00, Tim Deegan wrote:
> > 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. :)
> 
> That is my point.
> 
> If 1GB pages are not supported (i.e. not present in cpuid), then the PS
> bit in an L3 is reserved, must be 0, and cause a pagefault if used.
> 
> Nothing in Xen currently enforces this because
> guest_supports_1G_superpages() doesn't check domain_cpuid().

For shadow mode, Xen DTRT by checking hvm_pse1gb_supported() in the
HVM cpuid handler, so we don't advertise a feature we can't support.

For HAP mode, we _could_ add a cpuid check to the pagetable walker
but...

> It does however make me wonder how VMX/SVM non-root mode would enforce
> this as it would see the host cpuid, not guest cpuid when performing
> paging internally.

...non-emulated PT walks will get to use 1GB superpages anyway.
This is the same for other features (new instructions &c).  We
can mask them out of CPUID but by and large we can't make them fault.

Tim.

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