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

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Message-ID: <a1a82bc0-9a7a-5363-cda8-a0226eff0073@linux.intel.com>
Date:   Tue, 18 Apr 2023 10:06:12 +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 4/18/23 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.

Best regards,
baolu