Re: [PATCH 1/4] riscv: tlb: fix __p*d_free_tlb()

[Alexandre Ghiti]

Hi Jisheng,

On 31/12/2023 07:21, Alexandre Ghiti wrote:
> Hi Jisheng,
>
> On 19/12/2023 18:50, Jisheng Zhang wrote:
> > If non-leaf PTEs I.E pmd, pud or p4d is modified, a sfence.vma is
> > a must for safe, imagine if an implementation caches the non-leaf
> > translation in TLB, although I didn't meet this HW so far, but it's
> > possible in theory.
> >
> > Signed-off-by: Jisheng Zhang<jszhang@xxxxxxxxxx>
> > ---
> >   arch/riscv/include/asm/pgalloc.h | 20 +++++++++++++++++---
> >   1 file changed, 17 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/riscv/include/asm/pgalloc.h 
> > b/arch/riscv/include/asm/pgalloc.h
> > index d169a4f41a2e..a12fb83fa1f5 100644
> > --- a/arch/riscv/include/asm/pgalloc.h
> > +++ b/arch/riscv/include/asm/pgalloc.h
> > @@ -95,7 +95,13 @@ static inline void pud_free(struct mm_struct *mm, 
> > pud_t *pud)
> >           __pud_free(mm, pud);
> >   }
> >   -#define __pud_free_tlb(tlb, pud, addr) pud_free((tlb)->mm, pud)
> > +#define __pud_free_tlb(tlb, pud, addr)                    \
> > +do {                                    \
> > +    if (pgtable_l4_enabled) {                    \
> > +        pagetable_pud_dtor(virt_to_ptdesc(pud));        \
> > +        tlb_remove_page_ptdesc((tlb), virt_to_ptdesc(pud)); \
>
>
> The specification indeed states that an sfence.vma must be emitted 
> after a page directory modification. Your change is not enough though 
> since eventually tlb_flush() is called and in this function we should 
> add:
>
> if (tlb->freed_tables)
>     tlb_flush_mm();


I sent a patch for that here 
https://lore.kernel.org/linux-riscv/20240128120405.25876-1-alexghiti@xxxxxxxxxxxx/

You can add:

Reviewed-by: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx>

Thanks,

Alex


> otherwise we are not guaranteed that a "global" sfence.vma is called.
>
> Would you be able to benchmark this change and see the performance 
> impact?
>
> Thanks,
>
> Alex
>
>
> > +    }                                \
> > +} while (0)
> >     #define p4d_alloc_one p4d_alloc_one
> >   static inline p4d_t *p4d_alloc_one(struct mm_struct *mm, unsigned 
> > long addr)
> > @@ -124,7 +130,11 @@ static inline void p4d_free(struct mm_struct 
> > *mm, p4d_t *p4d)
> >           __p4d_free(mm, p4d);
> >   }
> >   -#define __p4d_free_tlb(tlb, p4d, addr) p4d_free((tlb)->mm, p4d)
> > +#define __p4d_free_tlb(tlb, p4d, addr)                    \
> > +do {                                    \
> > +    if (pgtable_l5_enabled)                        \
> > +        tlb_remove_page_ptdesc((tlb), virt_to_ptdesc(p4d)); \
> > +} while (0)
> >   #endif /* __PAGETABLE_PMD_FOLDED */
> >     static inline void sync_kernel_mappings(pgd_t *pgd)
> > @@ -149,7 +159,11 @@ static inline pgd_t *pgd_alloc(struct mm_struct 
> > *mm)
> >     #ifndef __PAGETABLE_PMD_FOLDED
> >   -#define __pmd_free_tlb(tlb, pmd, addr) pmd_free((tlb)->mm, pmd)
> > +#define __pmd_free_tlb(tlb, pmd, addr)                \
> > +do {                                \
> > +    pagetable_pmd_dtor(virt_to_ptdesc(pmd));        \
> > +    tlb_remove_page_ptdesc((tlb), virt_to_ptdesc(pmd));    \
> > +} while (0)
> >     #endif /* __PAGETABLE_PMD_FOLDED */