linux-kernel - Re: [PATCH v4 3/7] iommu: Support allocation of global PASIDs outside SVA

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Message-ID: <7e182125-f5ab-3201-e69d-b6c96eeede01@linux.intel.com>
Date:   Wed, 19 Apr 2023 10:40:46 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     baolu.lu@...ux.intel.com, "Tian, Kevin" <kevin.tian@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        Robin Murphy <robin.murphy@....com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Joerg Roedel <joro@...tes.org>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
        "vkoul@...nel.org" <vkoul@...nel.org>,
        Will Deacon <will@...nel.org>,
        David Woodhouse <dwmw2@...radead.org>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        "Zanussi, Tom" <tom.zanussi@...el.com>
Subject: Re: [PATCH v4 3/7] iommu: Support allocation of global PASIDs outside
 SVA

On 4/19/23 7:04 AM, Jacob Pan wrote:
> On Tue, 18 Apr 2023 10:06:12 +0800, Baolu Lu<baolu.lu@...ux.intel.com>
> wrote:
> 
>> On 4/18/23 12:46 AM, Jacob Pan wrote:
>>> On Wed, 12 Apr 2023 09:37:48 +0800, Baolu Lu<baolu.lu@...ux.intel.com>
>>> wrote:
>>>    
>>>> On 4/11/23 4:02 PM, Tian, Kevin wrote:
>>>>>> From: Jacob Pan<jacob.jun.pan@...ux.intel.com>
>>>>>> Sent: Saturday, April 8, 2023 2:06 AM
>>>>>> @@ -28,8 +26,8 @@ static int iommu_sva_alloc_pasid(struct mm_struct
>>>>>> *mm, ioasid_t min, ioasid_t ma
>>>>>>     		goto out;
>>>>>>     	}
>>>>>>
>>>>>> -	ret = ida_alloc_range(&iommu_global_pasid_ida, min, max,
>>>>>> GFP_KERNEL);
>>>>>> -	if (ret < min)
>>>>>> +	ret = iommu_alloc_global_pasid(min, max);
>>>>> I wonder whether this can take a device pointer so
>>>>> dev->iommu->max_pasids is enforced inside the alloc function.
>>>> Agreed. Instead of using the open code, it looks better to have a
>>>> helper like dev_iommu_max_pasids().
>>> yes, probably export dev_iommu_get_max_pasids(dev)?
>>>
>>> But if I understood Kevin correctly, he's also suggesting that the
>>> interface should be changed to iommu_alloc_global_pasid(dev), my
>>> concern is that how do we use this function to reserve RID_PASID which
>>> is not specific to a device?
>> Probably we can introduce a counterpart dev->iommu->min_pasids, so that
>> there's no need to reserve the RID_PASID. At present, we can set it to 1
>> in the core as ARM/AMD/Intel all treat PASID 0 as a special pasid.
>>
>> In the future, if VT-d supports using arbitrary number as RID_PASID for
>> any specific device, we can call iommu_alloc_global_pasid() for that
>> device.
>>
>> The device drivers don't know and don't need to know the range of viable
>> PASIDs, so the @min, @max parameters seem to be unreasonable.
> Sure, that is reasonable. Another question is whether global PASID
> allocation is always for a single device, if not I prefer to keep the
> current iommu_alloc_global_pasid() and add a wrapper
> iommu_alloc_global_pasid_dev(dev) to extract the @min, @max. OK?

No problem from the code perspective. But we only need one API.

We can now add the kAPI that we really need. In this series, the idxd
driver wants to allocate a global PASID for its kernel dma with pasid
purpose. So, iommu_alloc_global_pasid_dev() seems to be sufficient.

If, in the future, we will have a need to provide global pasid
allocation other than device drivers, we can easily add the variants.

Best regards,
baolu