diff mbox series

[1/3] arm: Rename PMD_ORDER to PMD_TABLE_ORDER

Message ID 20210715134612.809280-2-willy@infradead.org (mailing list archive)
State Accepted, archived
Headers show
Series Make PMD_ORDER generically available | expand

Commit Message

Matthew Wilcox (Oracle) July 15, 2021, 1:46 p.m. UTC 
This is the order of the page table allocation, not the order of a PMD.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 arch/arm/kernel/head.S | 34 +++++++++++++++++-----------------
 1 file changed, 17 insertions(+), 17 deletions(-)

Comments

Russell King (Oracle) July 15, 2021, 4:47 p.m. UTC | #1 
On Thu, Jul 15, 2021 at 02:46:10PM +0100, Matthew Wilcox (Oracle) wrote:
> This is the order of the page table allocation, not the order of a PMD.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
>  arch/arm/kernel/head.S | 34 +++++++++++++++++-----------------
>  1 file changed, 17 insertions(+), 17 deletions(-)
> 
> diff --git a/arch/arm/kernel/head.S b/arch/arm/kernel/head.S
> index 9eb0b4dbcc12..6da39a1d70ba 100644
> --- a/arch/arm/kernel/head.S
> +++ b/arch/arm/kernel/head.S
> @@ -38,10 +38,10 @@
>  #ifdef CONFIG_ARM_LPAE
>  	/* LPAE requires an additional page for the PGD */
>  #define PG_DIR_SIZE	0x5000
> -#define PMD_ORDER	3
> +#define PMD_TABLE_ORDER	3
>  #else
>  #define PG_DIR_SIZE	0x4000
> -#define PMD_ORDER	2
> +#define PMD_TABLE_ORDER	2

I think PMD_ENTRY_ORDER would make more sense here - this is the
power-of-2 of an individual PMD entry, not of the entire table.
Matthew Wilcox (Oracle) July 15, 2021, 6:10 p.m. UTC | #2 
On Thu, Jul 15, 2021 at 05:47:41PM +0100, Russell King (Oracle) wrote:
> On Thu, Jul 15, 2021 at 02:46:10PM +0100, Matthew Wilcox (Oracle) wrote:
> > This is the order of the page table allocation, not the order of a PMD.
> > -#define PMD_ORDER	3
> > +#define PMD_TABLE_ORDER	3
> >  #else
> >  #define PG_DIR_SIZE	0x4000
> > -#define PMD_ORDER	2
> > +#define PMD_TABLE_ORDER	2
> 
> I think PMD_ENTRY_ORDER would make more sense here - this is the
> power-of-2 of an individual PMD entry, not of the entire table.

But ... we have two kinds of PMD entries.  We have the direct entry that
points to a 1-16MB sized chunk of memory, and we have the table entry that
points to a 4k-32k chunk of memory that contains PTEs.  So I don't think
calling it 'entry' order actually disambiguates anything.  That's why
I went with 'table' -- I can't think of anything else to call it!
PMD_PTE_ARRAY_ORDER doesn't seem like an improvement to me ...
Russell King (Oracle) July 15, 2021, 6:37 p.m. UTC | #3 
On Thu, Jul 15, 2021 at 07:10:54PM +0100, Matthew Wilcox wrote:
> On Thu, Jul 15, 2021 at 05:47:41PM +0100, Russell King (Oracle) wrote:
> > On Thu, Jul 15, 2021 at 02:46:10PM +0100, Matthew Wilcox (Oracle) wrote:
> > > This is the order of the page table allocation, not the order of a PMD.
> > > -#define PMD_ORDER	3
> > > +#define PMD_TABLE_ORDER	3
> > >  #else
> > >  #define PG_DIR_SIZE	0x4000
> > > -#define PMD_ORDER	2
> > > +#define PMD_TABLE_ORDER	2
> > 
> > I think PMD_ENTRY_ORDER would make more sense here - this is the
> > power-of-2 of an individual PMD entry, not of the entire table.
> 
> But ... we have two kinds of PMD entries.  We have the direct entry that
> points to a 1-16MB sized chunk of memory, and we have the table entry that
> points to a 4k-32k chunk of memory that contains PTEs.  So I don't think
> calling it 'entry' order actually disambiguates anything.  That's why
> I went with 'table' -- I can't think of anything else to call it!
> PMD_PTE_ARRAY_ORDER doesn't seem like an improvement to me ...

There may be two kinds of PMD entries, but that isn't relevant here.
Going back to the original terminology, 1 << PMD_ORDER here is the
size of each PMD entry. It doesn't have anything to do with how much
memory is being mapped by each entry.

I think what is confusing you is stuff like:

        add     r0, r4, #KERNEL_OFFSET >> (SECTION_SHIFT - PMD_ORDER)

r4 is the base address of the page tables, and r0 is the address of
the entry we want to manipulate for "KERNEL_OFFSET" - which is the
virtual address. 1 << SECTION_SHIFT is how much memory each entry
maps (and this is fixed here - there's no variability as you suggest
above.)

Effectively, the calculation above is:

	index = KERNEL_OFFSET >> SECTION_SHIFT;
	pmd_entry_size = 1 << PMD_ORDER;
	r0 = base + index * pmd_entry_size;

but in a single instruction as we can be sure that KERNEL_OFFSET will
have zeros as the low bits after shifting by SECTION_SHIFT - PMD_ORDER.

Hope this helps to explain what this PMD_ORDER is actually doing here.
Matthew Wilcox (Oracle) July 15, 2021, 7:16 p.m. UTC | #4 
On Thu, Jul 15, 2021 at 07:37:27PM +0100, Russell King (Oracle) wrote:
> On Thu, Jul 15, 2021 at 07:10:54PM +0100, Matthew Wilcox wrote:
> > On Thu, Jul 15, 2021 at 05:47:41PM +0100, Russell King (Oracle) wrote:
> > > On Thu, Jul 15, 2021 at 02:46:10PM +0100, Matthew Wilcox (Oracle) wrote:
> > > > This is the order of the page table allocation, not the order of a PMD.
> > > > -#define PMD_ORDER	3
> > > > +#define PMD_TABLE_ORDER	3
> > > >  #else
> > > >  #define PG_DIR_SIZE	0x4000
> > > > -#define PMD_ORDER	2
> > > > +#define PMD_TABLE_ORDER	2
> > > 
> > > I think PMD_ENTRY_ORDER would make more sense here - this is the
> > > power-of-2 of an individual PMD entry, not of the entire table.
> > 
> > But ... we have two kinds of PMD entries.  We have the direct entry that
> > points to a 1-16MB sized chunk of memory, and we have the table entry that
> > points to a 4k-32k chunk of memory that contains PTEs.  So I don't think
> > calling it 'entry' order actually disambiguates anything.  That's why
> > I went with 'table' -- I can't think of anything else to call it!
> > PMD_PTE_ARRAY_ORDER doesn't seem like an improvement to me ...
> 
> There may be two kinds of PMD entries, but that isn't relevant here.
> Going back to the original terminology, 1 << PMD_ORDER here is the
> size of each PMD entry. It doesn't have anything to do with how much
> memory is being mapped by each entry.

Oh.  Oh!  So, 'order' is usually a shift that is _added on to_ the
PAGE_SHIFT in order to find how many bytes are in question.  See
include/asm-generic/getorder.h.

Now, PMD_SHIFT is already in use, but perhaps what is meant here is
PMD_ENTRY_SHIFT?

> I think what is confusing you is stuff like:
> 
>         add     r0, r4, #KERNEL_OFFSET >> (SECTION_SHIFT - PMD_ORDER)
> 
> r4 is the base address of the page tables, and r0 is the address of
> the entry we want to manipulate for "KERNEL_OFFSET" - which is the
> virtual address. 1 << SECTION_SHIFT is how much memory each entry
> maps (and this is fixed here - there's no variability as you suggest
> above.)

(the variability I intended above was more to accommodate architectural
differences; I hate to use x86-specific numbers like 4KiB and 2MiB)

> Effectively, the calculation above is:
> 
> 	index = KERNEL_OFFSET >> SECTION_SHIFT;
> 	pmd_entry_size = 1 << PMD_ORDER;
> 	r0 = base + index * pmd_entry_size;
> 
> but in a single instruction as we can be sure that KERNEL_OFFSET will
> have zeros as the low bits after shifting by SECTION_SHIFT - PMD_ORDER.
> 
> Hope this helps to explain what this PMD_ORDER is actually doing here.

Thank you, yes, I was terminally confused.
diff mbox series

Patch

diff --git a/arch/arm/kernel/head.S b/arch/arm/kernel/head.S
index 9eb0b4dbcc12..6da39a1d70ba 100644
--- a/arch/arm/kernel/head.S
+++ b/arch/arm/kernel/head.S
@@ -38,10 +38,10 @@ 
 #ifdef CONFIG_ARM_LPAE
 	/* LPAE requires an additional page for the PGD */
 #define PG_DIR_SIZE	0x5000
-#define PMD_ORDER	3
+#define PMD_TABLE_ORDER	3
 #else
 #define PG_DIR_SIZE	0x4000
-#define PMD_ORDER	2
+#define PMD_TABLE_ORDER	2
 #endif
 
 	.globl	swapper_pg_dir
@@ -237,7 +237,7 @@  __create_page_tables:
 	mov	r6, r6, lsr #SECTION_SHIFT
 
 1:	orr	r3, r7, r5, lsl #SECTION_SHIFT	@ flags + kernel base
-	str	r3, [r4, r5, lsl #PMD_ORDER]	@ identity mapping
+	str	r3, [r4, r5, lsl #PMD_TABLE_ORDER]	@ identity mapping
 	cmp	r5, r6
 	addlo	r5, r5, #1			@ next section
 	blo	1b
@@ -247,13 +247,13 @@  __create_page_tables:
 	 * set two variables to indicate the physical start and end of the
 	 * kernel.
 	 */
-	add	r0, r4, #KERNEL_OFFSET >> (SECTION_SHIFT - PMD_ORDER)
+	add	r0, r4, #KERNEL_OFFSET >> (SECTION_SHIFT - PMD_TABLE_ORDER)
 	ldr	r6, =(_end - 1)
 	adr_l	r5, kernel_sec_start		@ _pa(kernel_sec_start)
 	str	r8, [r5]			@ Save physical start of kernel
 	orr	r3, r8, r7			@ Add the MMU flags
-	add	r6, r4, r6, lsr #(SECTION_SHIFT - PMD_ORDER)
-1:	str	r3, [r0], #1 << PMD_ORDER
+	add	r6, r4, r6, lsr #(SECTION_SHIFT - PMD_TABLE_ORDER)
+1:	str	r3, [r0], #1 << PMD_TABLE_ORDER
 	add	r3, r3, #1 << SECTION_SHIFT
 	cmp	r0, r6
 	bls	1b
@@ -269,14 +269,14 @@  __create_page_tables:
 	mov	r3, pc
 	mov	r3, r3, lsr #SECTION_SHIFT
 	orr	r3, r7, r3, lsl #SECTION_SHIFT
-	add	r0, r4,  #(XIP_START & 0xff000000) >> (SECTION_SHIFT - PMD_ORDER)
-	str	r3, [r0, #((XIP_START & 0x00f00000) >> SECTION_SHIFT) << PMD_ORDER]!
+	add	r0, r4,  #(XIP_START & 0xff000000) >> (SECTION_SHIFT - PMD_TABLE_ORDER)
+	str	r3, [r0, #((XIP_START & 0x00f00000) >> SECTION_SHIFT) << PMD_TABLE_ORDER]!
 	ldr	r6, =(_edata_loc - 1)
-	add	r0, r0, #1 << PMD_ORDER
-	add	r6, r4, r6, lsr #(SECTION_SHIFT - PMD_ORDER)
+	add	r0, r0, #1 << PMD_TABLE_ORDER
+	add	r6, r4, r6, lsr #(SECTION_SHIFT - PMD_TABLE_ORDER)
 1:	cmp	r0, r6
 	add	r3, r3, #1 << SECTION_SHIFT
-	strls	r3, [r0], #1 << PMD_ORDER
+	strls	r3, [r0], #1 << PMD_TABLE_ORDER
 	bls	1b
 #endif
 
@@ -286,10 +286,10 @@  __create_page_tables:
 	 */
 	mov	r0, r2, lsr #SECTION_SHIFT
 	cmp	r2, #0
-	ldrne	r3, =FDT_FIXED_BASE >> (SECTION_SHIFT - PMD_ORDER)
+	ldrne	r3, =FDT_FIXED_BASE >> (SECTION_SHIFT - PMD_TABLE_ORDER)
 	addne	r3, r3, r4
 	orrne	r6, r7, r0, lsl #SECTION_SHIFT
-	strne	r6, [r3], #1 << PMD_ORDER
+	strne	r6, [r3], #1 << PMD_TABLE_ORDER
 	addne	r6, r6, #1 << SECTION_SHIFT
 	strne	r6, [r3]
 
@@ -308,7 +308,7 @@  __create_page_tables:
 	addruart r7, r3, r0
 
 	mov	r3, r3, lsr #SECTION_SHIFT
-	mov	r3, r3, lsl #PMD_ORDER
+	mov	r3, r3, lsl #PMD_TABLE_ORDER
 
 	add	r0, r4, r3
 	mov	r3, r7, lsr #SECTION_SHIFT
@@ -338,7 +338,7 @@  __create_page_tables:
 	 * If we're using the NetWinder or CATS, we also need to map
 	 * in the 16550-type serial port for the debug messages
 	 */
-	add	r0, r4, #0xff000000 >> (SECTION_SHIFT - PMD_ORDER)
+	add	r0, r4, #0xff000000 >> (SECTION_SHIFT - PMD_TABLE_ORDER)
 	orr	r3, r7, #0x7c000000
 	str	r3, [r0]
 #endif
@@ -348,10 +348,10 @@  __create_page_tables:
 	 * Similar reasons here - for debug.  This is
 	 * only for Acorn RiscPC architectures.
 	 */
-	add	r0, r4, #0x02000000 >> (SECTION_SHIFT - PMD_ORDER)
+	add	r0, r4, #0x02000000 >> (SECTION_SHIFT - PMD_TABLE_ORDER)
 	orr	r3, r7, #0x02000000
 	str	r3, [r0]
-	add	r0, r4, #0xd8000000 >> (SECTION_SHIFT - PMD_ORDER)
+	add	r0, r4, #0xd8000000 >> (SECTION_SHIFT - PMD_TABLE_ORDER)
 	str	r3, [r0]
 #endif
 #endif