Re: [PATCH V3 09/11] x86/hyperv: Implement Hyper-V virtual IOMMU

[Mukesh R <mrathor@linux.microsoft.com>]

On 5/15/26 11:23, Jason Gunthorpe wrote:
> On Mon, May 11, 2026 at 07:02:57PM -0700, Mukesh R wrote:
>> +static struct iommu_domain *hv_iommu_domain_alloc_paging(struct device *dev)
>> +{
>> +	struct hv_domain *hvdom;
>> +	int rc;
>> +
>> +	if (hv_l1vh_partition() && !hv_curr_thread_is_vmm()) {
>> +		pr_err("Hyper-V: l1vh iommu does not support host devices\n");
>> +		return NULL;
>> +	}
>> +
>> +	hvdom = kzalloc(sizeof(struct hv_domain), GFP_KERNEL);
>> +	if (hvdom == NULL)
>> +		return NULL;
>> +
>> +	spin_lock_init(&hvdom->mappings_lock);
>> +	hvdom->mappings_tree = RB_ROOT_CACHED;
>> +
>> +	/* Called under iommu group mutex, so single threaded */
>> +	if (++unique_id == HV_DEVICE_DOMAIN_ID_S2_NULL)   /* ie, UINTMAX */
>> +		goto out_err;
>> +
>> +	hvdom->domid_num = unique_id;
>> +	hvdom->partid = hv_get_current_partid();
>> +	hvdom->iommu_dom.geometry = default_geometry;
>> +	hvdom->iommu_dom.pgsize_bitmap = HV_IOMMU_PGSIZES;
>> +
>> +	/* For guests, by default we do direct attaches, so no domain in hyp */
>> +	if (hv_dom_owner_is_vmm(hvdom) && !hv_no_attdev)
>> +		hvdom->attached_dom = true;
> 
> What are you thinking sending something like this?!?!?
> 
> The function is called *alloc domain PAGING*, it does not, and can not
> allocate weird "special" domains that are not PAGING domains. I just
> spent a long time removing all this kind of crazyness from drivers.
> 
> There is alot of other things I don't like in this patch, but this is
> too much.
> 
> You have to drop this "direct attach" idea from the first iteration,
> Linux can't do it without alot more work, you should start with the
> basic paging domain mode.
> 
> Jason

Yeah, agree. There was some ambivalence whether this could be a stop gap
solution until the iommufd based solution is fully designed.

I'll remove the "direct attach" stuff and resend with just basic paging
domain mode. Thanks for the review.

Thanks,
-Mukesh