Re: [PATCH v2 09/17] iommufd: Add enforced_cache_coherency to iommufd_hw_pagetable_alloc()

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

On 3/8/23 11:06 PM, Jason Gunthorpe wrote:
> On Wed, Mar 08, 2023 at 09:04:47PM +0800, Baolu Lu wrote:
>>> +	/* Try to upgrade the domain we have */
>>> +	if (idev->enforce_cache_coherency) {
>>> +		rc = iommufd_hw_pagetable_enforce_cc(hwpt);
>>> +		if (rc)
>>> +			return rc;
>> As "HWPT should have the coherency set properly for the device that it
>> is being created for when it is created", is it an incompatible case if
>>
>> 	idev->enforce_cache_coherency && !hwpt->enforce_cache_coherency
>>
>> ?
>>
>> If so, why not,
>>
>> 	if (idev->enforce_cache_coherency && !hwpt->enforce_cache_coherency)
>> 		return -EINVAL;
> In principle you can create a HWPT for device A and then later attach
> it to device B
> 
> If B requires coherency then we try to upgrade the existing domain.
> 
> This is an effort to try an minimize the number of domains that we
> need to have for simpe IOAS based cases where we don't want to spawn
> two auto domains

OK. Clear to me now. Actually hwpt->enforce_cache_coherency makes sense
only after it is attached to an idev.

Best regards,
baolu