From mboxrd@z Thu Jan 1 00:00:00 1970 From: Alexander Graf Date: Tue, 06 May 2014 16:18:10 +0000 Subject: Re: [RFC PATCH] KVM: PPC: BOOK3S: HV: THP support for guest Message-Id: <53690B42.1090707@suse.de> List-Id: References: <1399224616-25142-1-git-send-email-aneesh.kumar@linux.vnet.ibm.com> <5368A78D.4070509@suse.de> <1399368400.18906.9.camel@pasglop> <5368ADE3.1050503@suse.de> <87oazbq73t.fsf@linux.vnet.ibm.com> <5368FE66.5040809@suse.de> <87ha52ritd.fsf@linux.vnet.ibm.com> In-Reply-To: <87ha52ritd.fsf@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: "Aneesh Kumar K.V" Cc: paulus@samba.org, linuxppc-dev@lists.ozlabs.org, kvm-ppc@vger.kernel.org, kvm@vger.kernel.org On 05/06/2014 06:08 PM, Aneesh Kumar K.V wrote: > Alexander Graf writes: > >> On 05/06/2014 05:06 PM, Aneesh Kumar K.V wrote: >>> Alexander Graf 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