linux-kernel - Re: [PATCH v2 4/8] iommu/vt-d: Reserve RID

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Date:   Wed, 29 Mar 2023 14:13:14 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     baolu.lu@...ux.intel.com, LKML <linux-kernel@...r.kernel.org>,
        iommu@...ts.linux.dev, Jason Gunthorpe <jgg@...dia.com>,
        Joerg Roedel <joro@...tes.org>, dmaengine@...r.kernel.org,
        vkoul@...nel.org, Robin Murphy <robin.murphy@....com>,
        Will Deacon <will@...nel.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Raj Ashok <ashok.raj@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>, Yi Liu <yi.l.liu@...el.com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        "Zanussi, Tom" <tom.zanussi@...el.com>
Subject: Re: [PATCH v2 4/8] iommu/vt-d: Reserve RID_PASID from global SVA
 PASID space

On 3/29/23 4:52 AM, Jacob Pan wrote:
> On Tue, 28 Mar 2023 09:29:19 -0700, Jacob Pan
> <jacob.jun.pan@...ux.intel.com>  wrote:
> 
>>>> On VT-d platforms, RID_PASID is used for DMA request without PASID. We
>>>> should not treat RID_PASID special instead let it be allocated from
>>>> the global SVA PASID number space.
>>> It's same to AMD and ARM SMMUv3, right? They also need an explicit
>>> reservation of PASID 0.
>>>    
>> yes, all IOMMU drivers need to do that. I will give it a try but might
>> need help to place the call.
> It might be simpler to just let SVA code allocate from 1 up instead of 0
> (as is in the current code). Global PASID allocator would still allow the
> full range from 0 to max. Then there is no change to architectures that
> don't support non-zero RID_PASID. For VT-d, it would still work in the
> future when we have nonzero RID_PASID. is that reasonable?

Yes. It's reasonable from the status quo.

Best regards,
baolu