Re: [PATCH v2 11/16] iommu/vt-d: preserve PASID table of preserved device

[Baolu Lu <baolu.lu@linux.intel.com>]

On 5/12/2026 2:45 AM, Samiullah Khawaja wrote:
> On Fri, May 08, 2026 at 02:05:58PM +0800, Baolu Lu wrote:
>> On 4/28/26 01:56, Samiullah Khawaja wrote:
>>> In scalable mode the PASID table is used to fetch the io page tables.
>>> Preserve and restore the PASID table of the preserved devices.
>>>
>>> Signed-off-by: Samiullah Khawaja<skhawaja@google.com>
>>> ---
>>>  drivers/iommu/intel/iommu.c      |   5 +-
>>>  drivers/iommu/intel/iommu.h      |  12 +++
>>>  drivers/iommu/intel/liveupdate.c | 141 +++++++++++++++++++++++++++++++
>>>  drivers/iommu/intel/pasid.c      |   7 +-
>>>  drivers/iommu/intel/pasid.h      |   9 ++
>>>  include/linux/kho/abi/iommu.h    |  13 +++
>>>  6 files changed, 184 insertions(+), 3 deletions(-)
>>>
> 
> [snip]
>>> +
>>> +void pasid_cleanup_preserved_table(struct device *dev)
>>> +{
>>> +    struct pasid_table *pasid_table;
>>> +    struct pasid_dir_entry *dir;
>>> +    struct pasid_entry *table;
>>> +    size_t dir_size;
>>> +
>>> +    pasid_table = intel_pasid_get_table(dev);
>>> +    if (!pasid_table)
>>> +        return;
>>> +
>>> +    dir = pasid_table->table;
>>> +    table = get_pasid_table_from_pde(&dir[0]);
>>> +    if (!table)
>>> +        return;
>>> +
>>> +    /* Clear everything except the first entry in table. */
>>> +    memset(&table[1], 0, SZ_4K - sizeof(*table));
>>> +
>>> +    /* Use the folio order to calculate the size of Pasid Directory */
>>> +    dir_size = (1 << (folio_order(virt_to_folio(dir)) + PAGE_SHIFT));
>>> +
>>> +    /* Clear everything except the first entry in directory */
>>> +    memset(&dir[1], 0, dir_size - sizeof(struct pasid_dir_entry));
>>> +
>>> +    clflush_cache_range(&table[0], SZ_4K);
>>> +    clflush_cache_range(&dir[0], dir_size);
>>> +}
>>
>> The PASID table is currently active and in use by the hardware. Clearing
>> the entries without the necessary hardware cache invalidation is buggy.
>>
>> It seems this manual clearing is a workaround because PASID domain
>> preservation isn't supported yet. If so, rather than clearing the table
>> blindly, the code should verify if any PASIDs (other than
>> IOMMU_NO_PASID) are actually in use. If there are, the preserve callback
>> should return an error.
> 
> Thanks for looking into this, I agree and will remove the clearing logic
> here.
> 
> Yes, we do this check in iommufd, as it is the dma owner of the device,
> and only DMA owned devices are allowed to be preserved.
> 
> During preservation iommufd returns an error if the device has PASID
> (non NO_PASID) attachments. And once the device is preserved, any PASID
> attachments are not allowed until the device is unpreserved.
> 
> I think I will make this check robust by moving it into core and use
> pasid_array. It will require some plumbing as pasid_array exists in
> iommu.c file.

Yeah, that would be better.

Thanks,
baolu