Re: [PATCH v2 1/3] riscv: Improve PTDUMP to show RSW with non-zero value

[Yu-Chien Peter Lin]

Hi Alexandre,

On Fri, Sep 15, 2023 at 01:07:04PM +0200, Alexandre Ghiti wrote:
> On 14/09/2023 03:40, Yu Chien Peter Lin wrote:
> > RSW field can be used to encode 2 bits of software defined
> > information, currently PTDUMP only prints RSW when its value
> > is 1 or 3.
> > 
> > To fix this issue and enhance the debug experience with PTDUMP,
> > we use _PAGE_SOFT as the RSW mask and redefine _PAGE_SPECIAL to
> > (1 << 8), allow it to print the RSW with any non-zero value,
> > otherwise, it will print an empty string for each row.
> > 
> > This patch also removes the val from the struct prot_bits as
> > it is no longer needed.
> > 
> > Signed-off-by: Yu Chien Peter Lin <peterlin@xxxxxxxxxxxxx>
> > ---
> >   arch/riscv/include/asm/pgtable-bits.h |  4 +--
> >   arch/riscv/mm/ptdump.c                | 36 +++++++++++----------------
> >   2 files changed, 17 insertions(+), 23 deletions(-)
> > 
> > diff --git a/arch/riscv/include/asm/pgtable-bits.h b/arch/riscv/include/asm/pgtable-bits.h
> > index f896708e8331..99e60fd3eb72 100644
> > --- a/arch/riscv/include/asm/pgtable-bits.h
> > +++ b/arch/riscv/include/asm/pgtable-bits.h
> > @@ -16,9 +16,9 @@
> >   #define _PAGE_GLOBAL    (1 << 5)    /* Global */
> >   #define _PAGE_ACCESSED  (1 << 6)    /* Set by hardware on any access */
> >   #define _PAGE_DIRTY     (1 << 7)    /* Set by hardware on any write */
> > -#define _PAGE_SOFT      (1 << 8)    /* Reserved for software */
> > +#define _PAGE_SOFT      (3 << 8)    /* Reserved for software */
> > -#define _PAGE_SPECIAL   _PAGE_SOFT
> 
> 
> That's nit, but maybe you could have introduced a _PAGE_SOFT_1 and
> _PAGE_SOFT_2
> 

Thanks for the suggestion, maybe we just add a comment here?

#define _PAGE_SPECIAL   (1 << 8) /* RSW: 0x1 */

> > +#define _PAGE_SPECIAL   (1 << 8)
> 
> 
> instead of hardcoding (1<<8) here, but that can be done when we'll use the
> second bit :)
> 
> >   #define _PAGE_TABLE     _PAGE_PRESENT
> >   /*
> > diff --git a/arch/riscv/mm/ptdump.c b/arch/riscv/mm/ptdump.c
> > index 20a9f991a6d7..85686652f342 100644
> > --- a/arch/riscv/mm/ptdump.c
> > +++ b/arch/riscv/mm/ptdump.c
> > @@ -129,7 +129,6 @@ static struct ptd_mm_info efi_ptd_info = {
> >   /* Page Table Entry */
> >   struct prot_bits {
> >   	u64 mask;
> > -	u64 val;
> >   	const char *set;
> >   	const char *clear;
> >   };
> > @@ -137,47 +136,38 @@ struct prot_bits {
> >   static const struct prot_bits pte_bits[] = {
> >   	{
> >   		.mask = _PAGE_SOFT,
> > -		.val = _PAGE_SOFT,
> > -		.set = "RSW",
> > -		.clear = "   ",
> > +		.set = "RSW(%d)",
> > +		.clear = "      ",
> >   	}, {
> >   		.mask = _PAGE_DIRTY,
> > -		.val = _PAGE_DIRTY,
> >   		.set = "D",
> >   		.clear = ".",
> >   	}, {
> >   		.mask = _PAGE_ACCESSED,
> > -		.val = _PAGE_ACCESSED,
> >   		.set = "A",
> >   		.clear = ".",
> >   	}, {
> >   		.mask = _PAGE_GLOBAL,
> > -		.val = _PAGE_GLOBAL,
> >   		.set = "G",
> >   		.clear = ".",
> >   	}, {
> >   		.mask = _PAGE_USER,
> > -		.val = _PAGE_USER,
> >   		.set = "U",
> >   		.clear = ".",
> >   	}, {
> >   		.mask = _PAGE_EXEC,
> > -		.val = _PAGE_EXEC,
> >   		.set = "X",
> >   		.clear = ".",
> >   	}, {
> >   		.mask = _PAGE_WRITE,
> > -		.val = _PAGE_WRITE,
> >   		.set = "W",
> >   		.clear = ".",
> >   	}, {
> >   		.mask = _PAGE_READ,
> > -		.val = _PAGE_READ,
> >   		.set = "R",
> >   		.clear = ".",
> >   	}, {
> >   		.mask = _PAGE_PRESENT,
> > -		.val = _PAGE_PRESENT,
> >   		.set = "V",
> >   		.clear = ".",
> >   	}
> > @@ -208,15 +198,19 @@ static void dump_prot(struct pg_state *st)
> >   	unsigned int i;
> >   	for (i = 0; i < ARRAY_SIZE(pte_bits); i++) {
> > -		const char *s;
> > -
> > -		if ((st->current_prot & pte_bits[i].mask) == pte_bits[i].val)
> > -			s = pte_bits[i].set;
> > -		else
> > -			s = pte_bits[i].clear;
> > -
> > -		if (s)
> > -			pt_dump_seq_printf(st->seq, " %s", s);
> > +		char s[7];
> > +		unsigned long val;
> > +
> > +		val = st->current_prot & pte_bits[i].mask;
> > +		if (val) {
> > +			if (pte_bits[i].mask == _PAGE_SOFT)
> > +				sprintf(s, pte_bits[i].set, val >> 8);
> > +			else
> > +				sprintf(s, "%s", pte_bits[i].set);
> > +		} else
> > +			sprintf(s, "%s", pte_bits[i].clear);
> > +
> > +		pt_dump_seq_printf(st->seq, " %s", s);
> >   	}
> >   }
> 
> 
> I don't see any issue in your patch, but just the output is a bit "weird"
> now as the there is a large "hole" between the PTE type and the PTE
> protection bits:
> 
> Before:
> 
> 0xffffffd800000000-0xffffffd800200000 0x0000000080000000         2M PMD    
> D A G . . W R V
> 
> After:
> 
> 0xffffaf8000000000-0xffffaf8000200000 0x0000000080000000         2M PMD
> .               D A G . . W R V
> 
> Maybe you could add the PBMT/N bits after the protections bits to void this
> hole?

Agreed, PBMT and RSW fields are not commonly used. How about adding ".."
for 2-bit zero values instead of spaces? hopefully it will look better.

0xffffffc802088000-0xffffffc80208c000    0x0000000000d36000        16K PTE .   ..     ..   D A G . . W R V
0xffffffc802090000-0xffffffc802094000    0x0000000000d4d000        16K PTE . MT(IO)   ..   D A G . . W R V
0xffffffc802095000-0xffffffc8020b5000    0x0000000100d80000       128K PTE .   ..   RSW(2) D A G . . W R V
0xffffffc8020b6000-0xffffffc8020d6000    0x0000000100da0000       128K PTE .   ..   RSW(2) D A G . . W R V
0xffffffc8020d8000-0xffffffc8020dc000    0x0000000000d7b000        16K PTE .   ..     ..   D A G . . W R V
0xffffffc8020e0000-0xffffffc8020e4000    0x0000000000d7f000        16K PTE .   ..     ..   D A G . . W R V

> Anyway, as a heavy user of this kernel page table dump, that's really
> appreciated, thanks :)

Thansk for the review :)

Best regards,
Peter Lin

> Alex
>