linux-kernel - Re: [PATCH v3 4/7] iommu/vt-d: Reserve RID

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Date:   Sat, 1 Apr 2023 21:43:13 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>, iommu@...ts.linux.dev,
        Robin Murphy <robin.murphy@....com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Joerg Roedel <joro@...tes.org>, dmaengine@...r.kernel.org,
        vkoul@...nel.org
Cc:     baolu.lu@...ux.intel.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 v3 4/7] iommu/vt-d: Reserve RID_PASID from global PASID
 space

On 2023/4/1 7:11, Jacob Pan 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 PASID number space. Non-zero value can be used in RID_PASID on
> Intel VT-d.
> 
> For ARM, AMD and others that _always_ use 0 as RID_PASID, there is no
> impact in that SVA PASID allocation base is 1.
> 
> With this change, devices do both DMA with PASID and SVA will not worry
> about conflicts when it comes to allocating PASIDs for in-kernel DMA.
> 
> Signed-off-by: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> ---
>   drivers/iommu/intel/iommu.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index 9f737ef55463..cbb2670f88ca 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -3956,6 +3956,10 @@ int __init intel_iommu_init(void)
>   
>   	intel_iommu_enabled = 1;
>   
> +	/* Reserved RID_PASID from the global namespace for legacy DMA */
> +	WARN_ON(iommu_alloc_global_pasid(PASID_RID2PASID, PASID_RID2PASID) !=
> +		PASID_RID2PASID);

This looks odd. With the previous patch unchanged,
iommu_alloc_global_pasid() will always return IOMMU_PASID_INVALID if
CONFIG_IOMMU_SVA is not set.

> +
>   	return 0;
>   
>   out_free_dmar:

Best regards,
baolu