Re: [PATCH v2 17/18] AMD/IOMMU: free all-empty page tables

["Roger Pau Monné" <roger.pau@citrix.com>]

On Fri, Sep 24, 2021 at 11:55:57AM +0200, Jan Beulich wrote:
> When a page table ends up with no present entries left, it can be
> replaced by a non-present entry at the next higher level. The page table
> itself can then be scheduled for freeing.
> 
> Note that while its output isn't used there yet, update_contig_markers()
> right away needs to be called in all places where entries get updated,
> not just the one where entries get cleared.

Couldn't you also coalesce all contiguous page tables into a
super-page entry at the higher level? (not that should be done here,
it's just that I'm not seeing any patch to that effect in the series)

> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> ---
> v2: New.
> 
> --- a/xen/drivers/passthrough/amd/iommu_map.c
> +++ b/xen/drivers/passthrough/amd/iommu_map.c
> @@ -21,6 +21,9 @@
>  
>  #include "iommu.h"
>  
> +#define CONTIG_MASK IOMMU_PTE_CONTIG_MASK
> +#include <asm/contig-marker.h>
> +
>  /* Given pfn and page table level, return pde index */
>  static unsigned int pfn_to_pde_idx(unsigned long pfn, unsigned int level)
>  {
> @@ -33,16 +36,20 @@ static unsigned int pfn_to_pde_idx(unsig
>  
>  static union amd_iommu_pte clear_iommu_pte_present(unsigned long l1_mfn,
>                                                     unsigned long dfn,
> -                                                   unsigned int level)
> +                                                   unsigned int level,
> +                                                   bool *free)
>  {
>      union amd_iommu_pte *table, *pte, old;
> +    unsigned int idx = pfn_to_pde_idx(dfn, level);
>  
>      table = map_domain_page(_mfn(l1_mfn));
> -    pte = &table[pfn_to_pde_idx(dfn, level)];
> +    pte = &table[idx];
>      old = *pte;
>  
>      write_atomic(&pte->raw, 0);
>  
> +    *free = update_contig_markers(&table->raw, idx, level, PTE_kind_null);
> +
>      unmap_domain_page(table);
>  
>      return old;
> @@ -85,7 +92,11 @@ static union amd_iommu_pte set_iommu_pte
>      if ( !old.pr || old.next_level ||
>           old.mfn != next_mfn ||
>           old.iw != iw || old.ir != ir )
> +    {
>          set_iommu_pde_present(pde, next_mfn, 0, iw, ir);
> +        update_contig_markers(&table->raw, pfn_to_pde_idx(dfn, level), level,
> +                              PTE_kind_leaf);
> +    }
>      else
>          old.pr = false; /* signal "no change" to the caller */
>  
> @@ -259,6 +270,9 @@ static int iommu_pde_from_dfn(struct dom
>              smp_wmb();
>              set_iommu_pde_present(pde, next_table_mfn, next_level, true,
>                                    true);
> +            update_contig_markers(&next_table_vaddr->raw,
> +                                  pfn_to_pde_idx(dfn, level),
> +                                  level, PTE_kind_table);
>  
>              *flush_flags |= IOMMU_FLUSHF_modified;
>          }
> @@ -284,6 +298,9 @@ static int iommu_pde_from_dfn(struct dom
>                  next_table_mfn = mfn_x(page_to_mfn(table));
>                  set_iommu_pde_present(pde, next_table_mfn, next_level, true,
>                                        true);
> +                update_contig_markers(&next_table_vaddr->raw,
> +                                      pfn_to_pde_idx(dfn, level),
> +                                      level, PTE_kind_table);

Would be nice if we could pack the update_contig_markers in
set_iommu_pde_present (like you do for clear_iommu_pte_present).

Thanks, Roger.