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

Alexander Graf
2014-05-06T16:18:10Z

On 05/06/2014 06:08 PM, Aneesh Kumar K.V wrote:
> Alexander Graf <agraf@suse.de> writes:
>
>> On 05/06/2014 05:06 PM, Aneesh Kumar K.V wrote:
>>> Alexander Graf <agraf@suse.de> writes:
>>>
>>>> On 05/06/2014 11:26 AM, Benjamin Herrenschmidt wrote:
>>>>> On Tue, 2014-05-06 at 11:12 +0200, Alexander Graf 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.

But KVM doesn't care. KVM cares about MPSS. It's like saying "Support
fork()" in a subject line while your patch implements page faults.

>
>>>
>>> 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.

Ah, I see :).

>
>>> 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.

Awesome. Thanks a lot!

Alex

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