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

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Date:   Tue, 28 Mar 2023 13:52:00 -0700
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>
Cc:     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>,
        jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH v2 4/8] iommu/vt-d: Reserve RID_PASID from global SVA
 PASID space

Hi Baolu,

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?

Thanks,

Jacob