Re: [RFC/PATCH v3 00/16] KVM/s390: Hugetlbfs enablement

[Janosch Frank <frankja@linux.vnet.ibm.com>]

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On 14.02.2018 15:30, David Hildenbrand wrote:
> On 09.02.2018 10:34, Janosch Frank wrote:
>> Since the z10 s390 does support 1M pages, but whereas hugetlbfs
>> support was added quite fast, KVM always used standard 4k pages for
>> guest backings.
>>
>> This patchset adds full support for 1M huge page backings for s390
>> KVM guests. I.e. we also support VSIE (nested vms) for these guests
>> and are therefore able to run all combinations of backings for all
>> layers of guests.
>>
>> When running a VSIE guest in a huge page backed guest, we need to
>> split some huge pages to be able to set granular protection. This way
>> we avoid a prot/unprot cycle if prefixes and VSIE pages containing
>> level 3 gmap DAT tables share the same segment, as the prefix has to
>> be accessible at all times and the VSIE page has to be write
>> protected.
>>
>> Branch:
>> git://git.kernel.org/pub/scm/linux/kernel/git/kvms390/linux.git hlp_vsie
>> https://git.kernel.org/pub/scm/linux/kernel/git/kvms390/linux.git/log/?h=hlp_vsie
>>
> 
> A general proposal: We will have split PMDs with fake PGSTE. This is
> nasty but needed. I think we should hinder virtualization from making
> use of these. Just like we already do for vSIE.
> 
> Should we make the KVM_CAP_S390_HPAGE a configuration option?
> 
> Without it being set, don't allow mapping huge pages into the GMAP.
> Everything as usual.
> 
> With it being set (by user space when it thinks we need huge pages),
> allow mapping huge pages into the GMAP AND
> - Explicitly disable CMMA. Right now we trust on user space to do the
>   right thing. ecb2 &= ~ECB2_CMMA
> - Disable PFMFI -> ecb2 &= ~ECB2_PFMFI
> - Disable SKF by setting scb->ictl |= ICTL_ISKE | ICTL_SSKE | ICTL_RRBE
> 
> So user space has to explicitly indicate and allow huge pages. This will
> result in all instructions that touch the PGSTE getting intercepted, so
> we can properly work on the huge PMDs instead.

My only concern here is:
Can this coexist with the cpumodels in a coordinated way?


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