Re: [RFC PATCH] KVM: PPC: BOOK3S: HV: THP support for guest

Benjamin Herrenschmidt
2014-05-06T20:35:53Z

On Tue, 2014-05-06 at 21:38 +0530, Aneesh Kumar K.V wrote:

> >> I updated the commit message as below. Let me know if this is ok.
> >>
> >> KVM: PPC: BOOK3S: HV: THP support for guest
> >
> > This has nothing to do with THP.
>
> THP support in guest depend on KVM advertising MPSS feature. We already
> have rest of the changes needed to support transparent huge pages
> upstream. (We do support THP with PowerVM LPAR already). The primary
> motivation of this patch is to enable THP in powerkvm guest.

I would argue (nit picking, I know ... :-) that the subject should be
"Enable MPSS support for guests", and the description can then explain
that this allows Linux guests to use THP.

Cheers,
Ben.

> >
> >>
> >> On recent IBM Power CPUs, while the hashed page table is looked up using
> >> the page size from the segmentation hardware (i.e. the SLB), it is
> >> possible to have the HPT entry indicate a larger page size. Thus for
> >> example it is possible to put a 16MB page in a 64kB segment, but since
> >> the hash lookup is done using a 64kB page size, it may be necessary to
> >> put multiple entries in the HPT for a single 16MB page. This
> >> capability is called mixed page-size segment (MPSS). With MPSS,
> >> there are two relevant page sizes: the base page size, which is the
> >> size used in searching the HPT, and the actual page size, which is the
> >> size indicated in the HPT entry. [ Note that the actual page size is
> >> always >= base page size ].
> >>
> >> We advertise MPSS feature to guest only if the host CPU supports the
> >> same. We use "ibm,segment-page-sizes" device tree node to advertise
> >> the MPSS support. The penc encoding indicate whether we support
> >> a specific combination of base page size and actual page size
> >> in the same segment. It is also the value used in the L|LP encoding
> >> of HPTE entry.
> >>
> >> In-order to support MPSS in guest, KVM need to handle the below details
> >> * advertise MPSS via ibm,segment-page-sizes
> >> * Decode the base and actual page size correctly from the HPTE entry
> >> so that we know what we are dealing with in H_ENTER and and can do
> >
> > Which code path exactly changes for H_ENTER?
>
> There is no real code path changes. Any code path that use
> hpte_page_size() is impacted. We return actual page size there.
>
> >
> >> the appropriate TLB invalidation in H_REMOVE and evictions.
> >
> > Apart from the grammar (which is pretty broken for the part that is not
> > copied from Paul) and the subject line this sounds quite reasonable.
> >
>
> Wll try to fix.
>
> -aneesh