Re: [PATCH v2 1/3] mini-os: mm: introduce generic page table walk function

[Samuel Thibault <samuel.thibault@xxxxxxxxxxxx>]

Juergen Gross, le mar. 13 août 2024 15:41:56 +0200, a ecrit:
> In x86 mm code there are multiple instances of page table walks for
> different purposes.
> 
> Introduce a generic page table walker being able to cover the current
> use cases. It will be used for other cases in future, too.
> 
> The page table walker needs some per-level data, so add a table for
> that data. Merge it with the already existing pt_prot[] array.
> 
> Rewrite get_pgt() to use the new walker.
> 
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>

Reviewed-by: SAmuel Thibault <samuel.thibault@xxxxxxxxxxxx>

> ---
> V2:
> - add idx_from_va_lvl() helper (Samuel Thibault)
> ---
>  arch/x86/mm.c | 157 +++++++++++++++++++++++++++++++++++++-------------
>  1 file changed, 118 insertions(+), 39 deletions(-)
> 
> diff --git a/arch/x86/mm.c b/arch/x86/mm.c
> index 7ddf16e4..9849b985 100644
> --- a/arch/x86/mm.c
> +++ b/arch/x86/mm.c
> @@ -125,20 +125,30 @@ void arch_mm_preinit(void *p)
>  }
>  #endif
>  
> +static const struct {
> +    unsigned int shift;
> +    unsigned int entries;
> +    pgentry_t prot;
> +} ptdata[PAGETABLE_LEVELS + 1] = {
> +    { 0, 0, 0 },
> +    { L1_PAGETABLE_SHIFT, L1_PAGETABLE_ENTRIES, L1_PROT },
> +    { L2_PAGETABLE_SHIFT, L2_PAGETABLE_ENTRIES, L2_PROT },
> +    { L3_PAGETABLE_SHIFT, L3_PAGETABLE_ENTRIES, L3_PROT },
> +#if defined(__x86_64__)
> +    { L4_PAGETABLE_SHIFT, L4_PAGETABLE_ENTRIES, L4_PROT },
> +#endif
> +};
> +
> +static inline unsigned int idx_from_va_lvl(unsigned long va, unsigned int 
> lvl)
> +{
> +    return (va >> ptdata[lvl].shift) & (ptdata[lvl].entries - 1);
> +}
> +
>  /*
>   * Make pt_pfn a new 'level' page table frame and hook it into the page
>   * table at offset in previous level MFN (pref_l_mfn). pt_pfn is a guest
>   * PFN.
>   */
> -static pgentry_t pt_prot[PAGETABLE_LEVELS] = {
> -    L1_PROT,
> -    L2_PROT,
> -    L3_PROT,
> -#if defined(__x86_64__)
> -    L4_PROT,
> -#endif
> -};
> -
>  static void new_pt_frame(unsigned long *pt_pfn, unsigned long prev_l_mfn, 
>                           unsigned long offset, unsigned long level)
>  {   
> @@ -170,7 +180,7 @@ static void new_pt_frame(unsigned long *pt_pfn, unsigned 
> long prev_l_mfn,
>      mmu_updates[0].ptr = (tab[l2_table_offset(pt_page)] & PAGE_MASK) + 
>          sizeof(pgentry_t) * l1_table_offset(pt_page);
>      mmu_updates[0].val = (pgentry_t)pfn_to_mfn(*pt_pfn) << PAGE_SHIFT | 
> -        (pt_prot[level - 1] & ~_PAGE_RW);
> +        (ptdata[level].prot & ~_PAGE_RW);
>      
>      if ( (rc = HYPERVISOR_mmu_update(mmu_updates, 1, NULL, DOMID_SELF)) < 0 )
>      {
> @@ -183,7 +193,7 @@ static void new_pt_frame(unsigned long *pt_pfn, unsigned 
> long prev_l_mfn,
>      mmu_updates[0].ptr =
>          ((pgentry_t)prev_l_mfn << PAGE_SHIFT) + sizeof(pgentry_t) * offset;
>      mmu_updates[0].val = (pgentry_t)pfn_to_mfn(*pt_pfn) << PAGE_SHIFT |
> -        pt_prot[level];
> +        ptdata[level + 1].prot;
>  
>      if ( (rc = HYPERVISOR_mmu_update(mmu_updates, 1, NULL, DOMID_SELF)) < 0 
> ) 
>      {
> @@ -192,7 +202,7 @@ static void new_pt_frame(unsigned long *pt_pfn, unsigned 
> long prev_l_mfn,
>      }
>  #else
>      tab = mfn_to_virt(prev_l_mfn);
> -    tab[offset] = (*pt_pfn << PAGE_SHIFT) | pt_prot[level];
> +    tab[offset] = (*pt_pfn << PAGE_SHIFT) | ptdata[level + 1].prot;
>  #endif
>  
>      *pt_pfn += 1;
> @@ -202,6 +212,82 @@ static void new_pt_frame(unsigned long *pt_pfn, unsigned 
> long prev_l_mfn,
>  static mmu_update_t mmu_updates[L1_PAGETABLE_ENTRIES + 1];
>  #endif
>  
> +/*
> + * Walk recursively through all PTEs calling a specified function. The 
> function
> + * is allowed to change the PTE, the walker will follow the new value.
> + * The walk will cover the virtual address range [from_va .. to_va].
> + * The supplied function will be called with the following parameters:
> + * va: base virtual address of the area covered by the current PTE
> + * lvl: page table level of the PTE (1 = lowest level, PAGETABLE_LEVELS =
> + *      PTE in page table addressed by %cr3)
> + * is_leaf: true if PTE doesn't address another page table (it is either at
> + *          level 1, or invalid, or has its PSE bit set)
> + * pte: address of the PTE
> + * par: parameter, passed to walk_pt() by caller
> + * Return value of func() being non-zero will terminate walk_pt(), walk_pt()
> + * will return that value in this case, zero else.
> + */
> +static int walk_pt(unsigned long from_va, unsigned long to_va,
> +                   int (func)(unsigned long va, unsigned int lvl,
> +                              bool is_leaf, pgentry_t *pte, void *par),
> +                   void *par)
> +{
> +    unsigned int lvl = PAGETABLE_LEVELS;
> +    unsigned int ptindex[PAGETABLE_LEVELS + 1];
> +    unsigned long va = round_pgdown(from_va);
> +    unsigned long va_lvl;
> +    pgentry_t *tab[PAGETABLE_LEVELS + 1];
> +    pgentry_t *pte;
> +    bool is_leaf;
> +    int ret;
> +
> +    /* Start at top level page table. */
> +    tab[lvl] = pt_base;
> +    ptindex[lvl] = idx_from_va_lvl(va, lvl);
> +
> +    while ( va < (to_va | (PAGE_SIZE - 1)) )
> +    {
> +        pte = tab[lvl] + ptindex[lvl];
> +        is_leaf = (lvl == L1_FRAME) || (*pte & _PAGE_PSE) ||
> +                  !(*pte & _PAGE_PRESENT);
> +        va_lvl = va & ~((1UL << ptdata[lvl].shift) - 1);
> +        ret = func(va_lvl, lvl, is_leaf, pte, par);
> +        if ( ret )
> +            return ret;
> +
> +        /* PTE might have been modified by func(), reevaluate leaf state. */
> +        is_leaf = (lvl == L1_FRAME) || (*pte & _PAGE_PSE) ||
> +                  !(*pte & _PAGE_PRESENT);
> +
> +        if ( is_leaf )
> +        {
> +            /* Reached a leaf PTE. Advance to next page. */
> +            va += 1UL << ptdata[lvl].shift;
> +            ptindex[lvl]++;
> +
> +            /* Check for the need to traverse up again. */
> +            while ( ptindex[lvl] == ptdata[lvl].entries )
> +            {
> +                /* End of virtual address space? */
> +                if ( lvl == PAGETABLE_LEVELS )
> +                    return 0;
> +                /* Reached end of current page table, one level up. */
> +                lvl++;
> +                ptindex[lvl]++;
> +            }
> +        }
> +        else
> +        {
> +            /* Not a leaf, walk one level down. */
> +            lvl--;
> +            tab[lvl] = mfn_to_virt(pte_to_mfn(*pte));
> +            ptindex[lvl] = idx_from_va_lvl(va, lvl);
> +        }
> +    }
> +
> +    return 0;
> +}
> +
>  /*
>   * Build the initial pagetable.
>   */
> @@ -407,36 +493,29 @@ static void set_readonly(void *text, void *etext)
>  /*
>   * get the PTE for virtual address va if it exists. Otherwise NULL.
>   */
> -static pgentry_t *get_pgt(unsigned long va)
> +static int get_pgt_func(unsigned long va, unsigned int lvl, bool is_leaf,
> +                        pgentry_t *pte, void *par)
>  {
> -    unsigned long mfn;
> -    pgentry_t *tab;
> -    unsigned offset;
> +    pgentry_t **result;
>  
> -    tab = pt_base;
> -    mfn = virt_to_mfn(pt_base);
> +    if ( !(*pte & _PAGE_PRESENT) && lvl > L1_FRAME )
> +        return -1;
>  
> -#if defined(__x86_64__)
> -    offset = l4_table_offset(va);
> -    if ( !(tab[offset] & _PAGE_PRESENT) )
> -        return NULL;
> -    mfn = pte_to_mfn(tab[offset]);
> -    tab = mfn_to_virt(mfn);
> -#endif
> -    offset = l3_table_offset(va);
> -    if ( !(tab[offset] & _PAGE_PRESENT) )
> -        return NULL;
> -    mfn = pte_to_mfn(tab[offset]);
> -    tab = mfn_to_virt(mfn);
> -    offset = l2_table_offset(va);
> -    if ( !(tab[offset] & _PAGE_PRESENT) )
> -        return NULL;
> -    if ( tab[offset] & _PAGE_PSE )
> -        return &tab[offset];
> -    mfn = pte_to_mfn(tab[offset]);
> -    tab = mfn_to_virt(mfn);
> -    offset = l1_table_offset(va);
> -    return &tab[offset];
> +    if ( lvl > L1_FRAME && !(*pte & _PAGE_PSE) )
> +        return 0;
> +
> +    result = par;
> +    *result = pte;
> +
> +    return 0;
> +}
> +
> +static pgentry_t *get_pgt(unsigned long va)
> +{
> +    pgentry_t *tab = NULL;
> +
> +    walk_pt(va, va, get_pgt_func, &tab);
> +    return tab;
>  }
>  
>  
> -- 
> 2.43.0
> 

-- 
Samuel
<N> je déteste import
<N> parce que lorsque tu fais du python et que tu oublies le #!/bin/env python 
et que tu mets le fichier exécutable
<N> import est exécuté
 -+- #ens-mim - pourquoi mon script python change le curseur de la souris ?! -+-