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

[Andrew Cooper <andrew.cooper3@citrix.com>]

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@eu.citrix.com> 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.

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

~Andrew