linux-kernel - RE: [PATCH v4 3/6] iommu/sva: Stop using ioasid

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Message-ID: <BN9PR11MB5276832C74C9350C510752168CB49@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Wed, 8 Mar 2023 07:32:31 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Jason Gunthorpe <jgg@...dia.com>
CC:     Baolu Lu <baolu.lu@...ux.intel.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        Joerg Roedel <joro@...tes.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "X86 Kernel" <x86@...nel.org>, "bp@...en8.de" <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "corbet@....net" <corbet@....net>,
        "vkoul@...nel.org" <vkoul@...nel.org>,
        "dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        Robin Murphy <robin.murphy@....com>,
        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>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>
Subject: RE: [PATCH v4 3/6] iommu/sva: Stop using ioasid_set for SVA

> From: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> Sent: Wednesday, March 8, 2023 1:42 AM
> 
> Hi Jason,
> 
> On Fri, 3 Mar 2023 09:15:45 -0400, Jason Gunthorpe <jgg@...dia.com>
> wrote:
> 
> > On Fri, Mar 03, 2023 at 05:57:41PM +0800, Baolu Lu wrote:
> > > On 2023/3/3 17:32, Jean-Philippe Brucker wrote:
> > > > > I suppose the common thing is reserving some kind of special
> > > > > PASIDs.
> > > > Are you planning to use RID_PASID != 0 in VT-d?  Otherwise we could
> > > > just communicate min_pasid from the IOMMU driver the same way we
> do
> > > > max_pasid.
> > > >
> > > > Otherwise I guess re-introduce a lighter ioasid_alloc() that the IOMMU
> > > > driver calls to reserve PASID0/RID_PASID.
> > >
> > > Yes. We probably will use a non-zero RID_PASID in the future. An
> > > interface to reserve (or allocate) a PASID from iommu_global_pasid_ida
> > > should work then.
> >
> > Just allowing the driver to store XA_ZERO_ENTRY would be fine
> >
> So we provide APIs for both?
> 1. alloc a global PASID, returned by this API
> 2. try to reserve a global PASID given by the driver, i.e.
> 	xa_cmpxchg(&iommu_global_pasid_ida.xa, 2, NULL,
> XA_ZERO_ENTRY,
> 			 GFP_KERNEL);
> seems #1 is sufficient.
> 

No need for both. There will be on-demand allocation on this global
space so a reservation interface doesn't make sense.