linux-kernel - Re: [PATCH v7 03/11] iommu/vt-d: Add custom allocator for IOASID

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Date:   Mon, 28 Oct 2019 15:49:00 -0700
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>
Cc:     Lu Baolu <baolu.lu@...ux.intel.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        "David Woodhouse" <dwmw2@...radead.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        "Jonathan Cameron" <jic23@...nel.org>,
        Eric Auger <eric.auger@...hat.com>,
        jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH v7 03/11] iommu/vt-d: Add custom allocator for IOASID

On Fri, 25 Oct 2019 15:52:39 +0000
"Tian, Kevin" <kevin.tian@...el.com> wrote:

> > From: Lu Baolu [mailto:baolu.lu@...ux.intel.com]
> > Sent: Friday, October 25, 2019 10:39 PM
> > 
> > Hi,
> > 
> > On 10/25/19 2:40 PM, Tian, Kevin wrote:  
> > >>>> ioasid_register_allocator(&iommu->pasid_allocator);
> > >>>> +			if (ret) {
> > >>>> +				pr_warn("Custom PASID
> > >>>> allocator registeration failed\n");
> > >>>> +				/*
> > >>>> +				 * Disable scalable mode on
> > >>>> this IOMMU if there
> > >>>> +				 * is no custom allocator.
> > >>>> Mixing SM capable vIOMMU
> > >>>> +				 * and non-SM vIOMMU are not
> > >>>> supported.
> > >>>> +				 */
> > >>>> +				intel_iommu_sm = 0;  
> > >>> It's insufficient to disable scalable mode by only clearing
> > >>> intel_iommu_sm. The DMA_RTADDR_SMT bit in root entry has
> > >>> already  
> > >> been  
> > >>> set. Probably, you need to
> > >>>
> > >>> for each iommu
> > >>> 	clear DMA_RTADDR_SMT in root entry
> > >>>
> > >>> Alternatively, since vSVA is the only customer of this custom
> > >>> PASID allocator, is it possible to only disable SVA here?
> > >>>  
> > >> Yeah, I think disable SVA is better. We can still do gIOVA in
> > >> SM. I guess we need to introduce a flag for sva_enabled.  
> > > I'm not sure whether tying above logic to SVA is the right
> > > approach. If vcmd interface doesn't work, the whole SM mode
> > > doesn't make sense which is based on PASID-granular protection
> > > (SVA is only one usage atop). If the only remaining usage of SM
> > > is to map gIOVA using reserved PASID#0, then why not disabling SM
> > > and just fallback to legacy mode?
> > >
> > > Based on that I prefer to disabling the SM mode completely (better
> > > through an interface), and move the logic out of CONFIG_INTEL_
> > > IOMMU_SVM
> > >  
> > 
> > Unfortunately, it is dangerous to disable SM after boot. SM uses
> > different root/device contexts and pasid table formats. Disabling SM
> > after boot requires changing above from SM format into legacy
> > format.  
> 
> You are correct.
> 
> > 
> > Since ioasid registration failure is a rare case. How about moving
> > this part of code up to the early stage of intel_iommu_init() and
> > returning error if hardware present vcmd capability but software
> > fails to register a custom ioasid allocator?
> >   
> 
> It makes sense to me.
> 
sounds good to me too, the earlier the less to clean up.
> Thanks
> Kevin

[Jacob Pan]