linux-kernel - Re: [PATCH v12 07/17] iommu: Try to allocate blocking domain when probing device

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Date:   Tue, 30 Aug 2022 09:46:01 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     baolu.lu@...ux.intel.com, Joerg Roedel <joro@...tes.org>,
        Christoph Hellwig <hch@...radead.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Vinod Koul <vkoul@...nel.org>,
        Eric Auger <eric.auger@...hat.com>,
        Liu Yi L <yi.l.liu@...el.com>,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        Zhangfei Gao <zhangfei.gao@...aro.org>,
        Zhu Tony <tony.zhu@...el.com>, iommu@...ts.linux.dev,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v12 07/17] iommu: Try to allocate blocking domain when
 probing device

On 2022/8/30 01:27, Jason Gunthorpe wrote:
> On Mon, Aug 29, 2022 at 11:40:24AM +0800, Baolu Lu wrote:
>> On 2022/8/26 22:52, Jason Gunthorpe wrote:
>>> On Fri, Aug 26, 2022 at 08:11:31PM +0800, Lu Baolu wrote:
>>>> Allocate the blocking domain when probing devices if the driver supports
>>>> blocking domain allocation. Otherwise, revert to the previous behavior,
>>>> that is, use UNMANAGED domain instead when the blocking domain is needed.
>>>>
>>>> Signed-off-by: Lu Baolu<baolu.lu@...ux.intel.com>
>>>> Tested-by: Zhangfei Gao<zhangfei.gao@...aro.org>
>>>> Tested-by: Tony Zhu<tony.zhu@...el.com>
>>>> ---
>>>>    drivers/iommu/iommu.c | 29 +++++++++++++++++------------
>>>>    1 file changed, 17 insertions(+), 12 deletions(-)
>>> This seems like a lot of overhead to allocate these things for every
>>> group?
>>>
>>> Why not add a simple refcount on the blocking domain instead and
>>> allocate the domain on the pasid attach like we do for ownership?
>>
>> I am working towards implementing static instance of blocking domain for
>> each IOMMU driver, and then, there's no much overhead to allocate it in
>> the probing device path.
> 
> Well, I thought about that and I don't think we can get
> there in a short order.

Yes. Fair enough.

> Would rather you progress this series without
> getting entangled in such a big adventure

Agreed. I will drop this patch and add below code in the iommu
interface:

--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -3219,6 +3219,26 @@ int iommu_attach_device_pasid(struct iommu_domain 
*domain,
                 return -ENODEV;

         mutex_lock(&group->mutex);
+
+       /*
+        * The underlying IOMMU driver needs to support blocking domain
+        * allocation and the callback to block DMA transactions with a
+        * specific PASID.
+        */
+       if (!group->blocking_domain) {
+               group->blocking_domain = __iommu_domain_alloc(dev->bus,
+                               IOMMU_DOMAIN_BLOCKED);
+               if (!group->blocking_domain) {
+                       ret = -ENODEV;
+                       goto out_unlock;
+               }
+       }
+
+       if (!group->blocking_domain->ops->set_dev_pasid) {
+               ret = -EOPNOTSUPP;
+               goto out_unlock;
+       }
+
         curr = xa_cmpxchg(&group->pasid_array, pasid, NULL, domain, 
GFP_KERNEL);
         if (curr) {
                 ret = xa_err(curr) ? : -EBUSY;

Currently both ARM SMMUv3 and VT-d drivers use static blocking domain.
Hence I didn't use a refcount for blocking domain release here.

Best regards,
baolu