Re: [PATCH v2 2/4] arm: mm: reordering memory type table

[Miles Chen]

On Fri, 2020-10-23 at 11:16 +0100, Russell King - ARM Linux admin wrote:
> On Fri, Oct 23, 2020 at 05:14:35PM +0800, Miles Chen wrote:
> > From: Minchan Kim <minchan@xxxxxxxxxx>
> > 
> > To use bit 5 in page table as L_PTE_SPECIAL, we need a room for that.
> > It seems we don't need 4 bits for the memory type with ARMv6+.
> > If it's true, let's reorder bits to make bit 5 free.
> > 
> > We will use the bit for L_PTE_SPECIAL in next patch.
> > 
> > A note from Catalin in [1]:
> > "
> > > Anyway, on ARMv7 or ARMv6+LPAE, the non-shared device gets mapped to
> > > shared device in hardware. Looking through the arm32 code, it seems that
> > > MT_DEVICE_NONSHARED is used by arch/arm/mach-shmobile/setup-r8a7779.c
> > > and IIUC that's a v7 platform (R-Car H1, Cortex-A9). I think the above
> > > should be defined to L_PTE_MT_DEV_SHARED, unless I miss any place where
> > > DEV_NONSHARED is relevant on ARMv6 (adding Simon to confirm on shmbile).
> > "
> > 
> > [1] https://lore.kernel.org/patchwork/patch/986574/
> > 
> > Cc: Russell King <linux@xxxxxxxxxxxxxxx>
> > Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> > Cc: Will Deacon <will.deacon@xxxxxxx>
> > Cc: Steve Capper <steve.capper@xxxxxxxxxx>
> > Cc: Simon Horman <horms@xxxxxxxxxxxx>
> > Cc: Minchan Kim <minchan@xxxxxxxxxx>
> > Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> > Signed-off-by: Miles Chen <miles.chen@xxxxxxxxxxxx>
> > ---
> >  arch/arm/include/asm/pgtable-2level.h | 21 +++++++++++++++++----
> >  arch/arm/mm/proc-macros.S             |  4 ++--
> >  2 files changed, 19 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/arm/include/asm/pgtable-2level.h b/arch/arm/include/asm/pgtable-2level.h
> > index 27a8635abea0..cdcd55cca37d 100644
> > --- a/arch/arm/include/asm/pgtable-2level.h
> > +++ b/arch/arm/include/asm/pgtable-2level.h
> > @@ -161,14 +161,27 @@
> >  #define L_PTE_MT_BUFFERABLE	(_AT(pteval_t, 0x01) << 2)	/* 0001 */
> >  #define L_PTE_MT_WRITETHROUGH	(_AT(pteval_t, 0x02) << 2)	/* 0010 */
> >  #define L_PTE_MT_WRITEBACK	(_AT(pteval_t, 0x03) << 2)	/* 0011 */
> > +#define L_PTE_MT_DEV_SHARED	(_AT(pteval_t, 0x04) << 2)	/* 0100 */
> > +#define L_PTE_MT_VECTORS	(_AT(pteval_t, 0x05) << 2)	/* 0101 */
> >  #define L_PTE_MT_MINICACHE	(_AT(pteval_t, 0x06) << 2)	/* 0110 (sa1100, xscale) */
> >  #define L_PTE_MT_WRITEALLOC	(_AT(pteval_t, 0x07) << 2)	/* 0111 */
> > -#define L_PTE_MT_DEV_SHARED	(_AT(pteval_t, 0x04) << 2)	/* 0100 */
> > -#define L_PTE_MT_DEV_NONSHARED	(_AT(pteval_t, 0x0c) << 2)	/* 1100 */
> 
> Sorry, no, this isn't going to work.
> 
> The lower two bits of this (bits 2 and 3) are explicitly designed to fit
> the C and B bits used in older architectures. Changing L_PTE_MT_VECTORS
> from having value '11' to '01' changes the functionality on older CPUs.
> 

thanks for the comment.

Is is possible to find other order to fit this? e.g.,

+#define L_PTE_MT_VECTORS	(_AT(pteval_t, 0x07) << 2)	/* 0111 */

#define L_PTE_MT_WRITEALLOC	(_AT(pteval_t, 0x05) << 2)	/* 0101 */

or only allow this types for the new CPUs?


Miles