Re: [PATCH v4 1/3] iommu_pt: support small VA for AMDv1

[Jason Gunthorpe]

On Wed, Apr 01, 2026 at 02:43:55PM +0000, Ankit Soni wrote:
> When hardware/VM request a small VA limit, the generic page table code
> clears PT_FEAT_DYNAMIC_TOP. This later causes domain initialization to 
> fail with -EOPNOTSUPP.
> To properly enforce the domain VA limit, clamp amdv1pt_possible_sizes
> using the requested max_vasz_lg2.
> 
> Signed-off-by: Ankit Soni <Ankit.Soni@xxxxxxx>
> ---
>  drivers/iommu/generic_pt/fmt/amdv1.h | 4 +++-
>  drivers/iommu/generic_pt/iommu_pt.h  | 4 ----
>  2 files changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/iommu/generic_pt/fmt/amdv1.h b/drivers/iommu/generic_pt/fmt/amdv1.h
> index 3b2c41d9654d..bc04d482c12f 100644
> --- a/drivers/iommu/generic_pt/fmt/amdv1.h
> +++ b/drivers/iommu/generic_pt/fmt/amdv1.h
> @@ -156,6 +156,7 @@ static inline unsigned int amdv1pt_num_items_lg2(const struct pt_state *pts)
>  static inline pt_vaddr_t amdv1pt_possible_sizes(const struct pt_state *pts)
>  {
>  	unsigned int isz_lg2 = pt_table_item_lg2sz(pts);
> +	pt_vaddr_t raw;
>  
>  	if (!amdv1pt_can_have_leaf(pts))
>  		return 0;
> @@ -168,8 +169,9 @@ static inline pt_vaddr_t amdv1pt_possible_sizes(const struct pt_state *pts)
>  	 * 512GB Pages are not supported due to a hardware bug.
>  	 * Otherwise every power of two size is supported.
>  	 */
> -	return GENMASK_ULL(min(51, isz_lg2 + amdv1pt_num_items_lg2(pts) - 1),
> +	raw = GENMASK_ULL(min(51, isz_lg2 + amdv1pt_num_items_lg2(pts) - 1),
>  			   isz_lg2) & ~SZ_512G;
> +	return fvalog2_mod(raw, pts->range->common->max_vasz_lg2);
>  }
>  #define pt_possible_sizes amdv1pt_possible_sizes

This is a generic issue, I think it should be fixed like this instead:

@@ -1153,8 +1153,12 @@ static void NS(get_info)(struct pt_iommu *iommu_table,
                        pgsize_bitmap |= pt_possible_sizes(&pts);
        }
 
-       /* Hide page sizes larger than the maximum OA */
-       info->pgsize_bitmap = oalog2_mod(pgsize_bitmap, common->max_oasz_lg2);
+       /*
+        * Hide page sizes larger than the maximum. -1 because a whole table
+        * pgsize is not allowed
+        */
+       info->pgsize_bitmap = log2_mod(pgsize_bitmap, common->max_vasz_lg2 - 1);
+       info->pgsize_bitmap = oalog2_mod(info->pgsize_bitmap, common->max_oasz_lg2);
 }
 
pt_possible_sizes() always has to be masked with the pgsize_bitmap, so
fixing it here is enough to remove those sizes from all the other
places..

Then one of the tests isn't following that rule, so it needs a fix too:

@@ -438,6 +440,9 @@ static void test_lvl_possible_sizes(struct kunit *test, struct pt_state *pts,
 {
        unsigned int num_items_lg2 = safe_pt_num_items_lg2(pts);
        pt_vaddr_t pgsize_bitmap = pt_possible_sizes(pts);
+       /* Matches get_info() */
+       pt_vaddr_t limited_pgsize_bitmap =
+               log2_mod(pgsize_bitmap, pts->range->common->max_vasz_lg2 - 1);
        unsigned int isz_lg2 = pt_table_item_lg2sz(pts);
 
        if (!pt_can_have_leaf(pts)) {
@@ -448,7 +453,8 @@ static void test_lvl_possible_sizes(struct kunit *test, struct pt_state *pts,
        /* No bits for sizes that would be outside this table */
        KUNIT_ASSERT_EQ(test, log2_mod(pgsize_bitmap, isz_lg2), 0);
        KUNIT_ASSERT_EQ(
-               test, fvalog2_div(pgsize_bitmap, num_items_lg2 + isz_lg2), 0);
+               test,
+               fvalog2_div(limited_pgsize_bitmap, num_items_lg2 + isz_lg2), 0);
 
        /*
         * Non contiguous must be supported. AMDv1 has a HW bug where it does
@@ -463,8 +469,8 @@ static void test_lvl_possible_sizes(struct kunit *test, struct pt_state *pts,
        /* A contiguous entry should not span the whole table */
        if (num_items_lg2 + isz_lg2 != PT_VADDR_MAX_LG2)
                KUNIT_ASSERT_FALSE(
-                       test,
-                       pgsize_bitmap & log2_to_int(num_items_lg2 + isz_lg2));
+                       test, limited_pgsize_bitmap &
+                                     log2_to_int(num_items_lg2 + isz_lg2));
 }

And it should go in its own patch: "fix pgsize_bitmap calculation in
get_info for smaller vasz's"

Jason