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

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Message-ID: <51f5c170-2659-d76a-afbb-632b7a55586c@ghiti.fr>
Date:   Fri, 15 Sep 2023 13:07:04 +0200
From:   Alexandre Ghiti <alex@...ti.fr>
To:     Yu Chien Peter Lin <peterlin@...estech.com>,
        paul.walmsley@...ive.com, palmer@...belt.com,
        aou@...s.berkeley.edu, david@...hat.com, akpm@...ux-foundation.org,
        alexghiti@...osinc.com, bjorn@...osinc.com,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc:     ycliang@...estech.com
Subject: Re: [PATCH v2 1/3] riscv: Improve PTDUMP to show RSW with non-zero
 value

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@...estech.com>
> ---
>   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


> +#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?

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

Alex