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

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

Paul Mackerras <paulus@samba.org> writes:

> On Mon, May 05, 2014 at 08:17:00PM +0530, Aneesh Kumar K.V wrote:
>> Alexander Graf <agraf@suse.de> writes:
>> 
>> > On 05/04/2014 07:30 PM, Aneesh Kumar K.V wrote:
>> >> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
>> >
>> > No patch description, no proper explanations anywhere why you're doing 
>> > what. All of that in a pretty sensitive piece of code. There's no way 
>> > this patch can go upstream in its current form.
>> >
>> 
>> Sorry about being vague. Will add a better commit message. The goal is
>> to export MPSS support to guest if the host support the same. MPSS
>> support is exported via penc encoding in "ibm,segment-page-sizes". The
>> actual format can be found at htab_dt_scan_page_sizes. When the guest
>> memory is backed by hugetlbfs we expose the penc encoding the host
>> support to guest via kvmppc_add_seg_page_size. 
>
> In a case like this it's good to assume the reader doesn't know very
> much about Power CPUs, and probably isn't familiar with acronyms such
> as MPSS.  The patch needs an introductory paragraph explaining that 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.

I will update the commit message with the above details

-aneesh