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

From: Alexandre Ghiti
Date: Mon Sep 18 2023 - 13:30:37 EST

Hi Peter Lin,

On Sat, Sep 16, 2023 at 9:23 AM Yu-Chien Peter Lin
<peterlin@xxxxxxxxxxxxx> wrote:
>
> 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
>

Fine by me, I'll add my RB/TB on the next version!

Thanks,

Alex

> > 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
> >