Re: [PATCH v4 15/21] AMD/IOMMU: free all-empty page tables

[Jan Beulich <jbeulich@xxxxxxxx>]

On 10.05.2022 15:30, Roger Pau Monné wrote:
> On Mon, Apr 25, 2022 at 10:42:19AM +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,
>> pt_update_contig_markers() right away needs to be called in all places
>> where entries get updated, not just the one where entries get cleared.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Thanks.

>> @@ -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);
>> +        pt_update_contig_markers(&table->raw, pfn_to_pde_idx(dfn, level),
>> +                                 level, PTE_kind_leaf);
> 
> It would be better to call pt_update_contig_markers inside of
> set_iommu_pde_present, but that would imply changing the parameters
> passed to the function.  It's cumbersome (and error prone) to have to
> pair calls to set_iommu_pde_present() with pt_update_contig_markers().

Right, but then already the sheer number of parameters would become
excessive (imo).

>> @@ -474,8 +491,24 @@ int cf_check amd_iommu_unmap_page(
>>  
>>      if ( pt_mfn )
>>      {
>> +        bool free;
>> +
>>          /* Mark PTE as 'page not present'. */
>> -        old = clear_iommu_pte_present(pt_mfn, dfn_x(dfn), level);
>> +        old = clear_iommu_pte_present(pt_mfn, dfn_x(dfn), level, &free);
>> +
>> +        while ( unlikely(free) && ++level < hd->arch.amd.paging_mode )
>> +        {
>> +            struct page_info *pg = mfn_to_page(_mfn(pt_mfn));
>> +
>> +            if ( iommu_pde_from_dfn(d, dfn_x(dfn), level, &pt_mfn,
>> +                                    flush_flags, false) )
>> +                BUG();
>> +            BUG_ON(!pt_mfn);
>> +
>> +            clear_iommu_pte_present(pt_mfn, dfn_x(dfn), level, &free);
> 
> Not sure it's worth initializing free to false (at definition and
> before each call to clear_iommu_pte_present), just in case we manage
> to return early from clear_iommu_pte_present without having updated
> 'free'.

There's no such path now, so I'd view it as dead code to do so. If
necessary a patch introducing such an early exit would need to deal
with this. But even then I'd rather see this being dealt with right
in clear_iommu_pte_present().

Jan