linux-kernel - Re: [RFC PATCH v4 12/16] powerpc/e500: Encode hugepage size in PTE bits

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Date: Wed, 29 May 2024 12:15:57 +0200
From: Oscar Salvador <osalvador@...e.com>
To: Christophe Leroy <christophe.leroy@...roup.eu>
Cc: Oscar Salvador <osalvador@...e.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jason Gunthorpe <jgg@...dia.com>, Peter Xu <peterx@...hat.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [RFC PATCH v4 12/16] powerpc/e500: Encode hugepage size in PTE
 bits

On Wed, May 29, 2024 at 10:14:15AM +0000, Christophe Leroy wrote:
> 
> 
> Le 29/05/2024 à 12:09, Oscar Salvador a écrit :
> > On Wed, May 29, 2024 at 09:49:48AM +0000, Christophe Leroy wrote:
> >> Doesn't really matter if it's PUD or PMD at this point. On a 32 bits
> >> kernel it will be all PMD while on a 64 bits kernel it is both PMD and PUD.
> >>
> >> At the time being (as implemented with hugepd), Linux support 4M, 16M,
> >> 64M, 256M and 1G (Shifts 22, 24, 26, 28, 30)
> >>
> >> The hardware supports the following page sizes, and encodes them on 4
> >> bits allthough it is not directly a shift. Maybe it would be better to
> >> use that encoding after all:
> > 
> > I think so.
> > 
> >>
> >> 0001 4 Kbytes (Shift 12)
> >> 0010 16 Kbytes (Shift 14)
> >> 0011 64 Kbytes (Shift 16)
> >> 0100 256 Kbytes (Shift 18)
> >> 0101 1 Mbyte (Shift 20)
> >> 0110 4 Mbytes (Shift 22)
> >> 0111 16 Mbytes (Shift 24)
> >> 1000 64 Mbytes (Shift 26)
> >> 1001 256 Mbytes (Shift 28)
> >> 1010 1 Gbyte (e500v2 only) (Shift 30)
> >> 1011 4 Gbytes (e500v2 only) (Shift 32)
> > 
> > You say hugehages start at 2MB (shift 21), but you say that the smallest hugepage
> > Linux support is 4MB (shift 22).?
> > 
> > 
> 
> No I say PMD_SIZE is 2MB on e500 with 64 bits PTE and at the time being 
> Linux powerpc implementation for e500 supports sizes 4M, 16M, 64M, 256M 
> and 1G.

Got it. I got confused.


-- 
Oscar Salvador
SUSE Labs