linux-kernel - Re: [PATCH 3/7] iommu/vt-d: Fix PASID devTLB invalidation

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Date:   Mon, 29 Jun 2020 21:58:59 -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>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Eric Auger" <eric.auger@...hat.com>, jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH 3/7] iommu/vt-d: Fix PASID devTLB invalidation

On Tue, 30 Jun 2020 03:01:29 +0000
"Tian, Kevin" <kevin.tian@...el.com> wrote:

> > From: Lu Baolu <baolu.lu@...ux.intel.com>
> > Sent: Thursday, June 25, 2020 3:26 PM
> > 
> > On 2020/6/23 23:43, Jacob Pan wrote:  
> > > DevTLB flush can be used for both DMA request with and without
> > > PASIDs. The former uses PASID#0 (RID2PASID), latter uses non-zero
> > > PASID for SVA usage.
> > >
> > > This patch adds a check for PASID value such that devTLB flush
> > > with PASID is used for SVA case. This is more efficient in that
> > > multiple PASIDs can be used by a single device, when tearing down
> > > a PASID entry we shall flush only the devTLB specific to a PASID.
> > >
> > > Fixes: 6f7db75e1c46 ("iommu/vt-d: Add second level page table")  
> 
> btw is it really a fix? From the description it's more like an
> optimization...
> 
I guess it depends on how the issue is perceived. There is no
functional problem but the flush is too coarse w/o this patch.

> > > Signed-off-by: Jacob Pan <jacob.jun.pan@...ux.intel.com>
> > > ---
> > >   drivers/iommu/intel/pasid.c | 11 ++++++++++-
> > >   1 file changed, 10 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/iommu/intel/pasid.c
> > > b/drivers/iommu/intel/pasid.c index c81f0f17c6ba..3991a24539a1
> > > 100644 --- a/drivers/iommu/intel/pasid.c
> > > +++ b/drivers/iommu/intel/pasid.c
> > > @@ -486,7 +486,16 @@ devtlb_invalidation_with_pasid(struct  
> > intel_iommu *iommu,  
> > >   	qdep = info->ats_qdep;
> > >   	pfsid = info->pfsid;
> > >
> > > -	qi_flush_dev_iotlb(iommu, sid, pfsid, qdep, 0, 64 -
> > > VTD_PAGE_SHIFT);
> > > +	/*
> > > +	 * When PASID 0 is used, it indicates RID2PASID(DMA
> > > request w/o  
> > PASID),  
> > > +	 * devTLB flush w/o PASID should be used. For non-zero
> > > PASID under
> > > +	 * SVA usage, device could do DMA with multiple PASIDs.
> > > It is more
> > > +	 * efficient to flush devTLB specific to the PASID.
> > > +	 */
> > > +	if (pasid)  
> > 
> > How about
> > 
> > 	if (pasid == PASID_RID2PASID)
> > 		qi_flush_dev_iotlb(iommu, sid, pfsid, qdep, 0, 64 -
> > VTD_PAGE_SHIFT);
> > 	else
> > 		qi_flush_dev_iotlb_pasid(iommu, sid, pfsid, pasid,
> > qdep, 0, 64 -
> > VTD_PAGE_SHIFT);
> > 
> > ?
> > 
> > It makes the code more readable and still works even we reassign
> > another pasid for RID2PASID.
> > 
> > Best regards,
> > baolu
> >   
> > > +		qi_flush_dev_iotlb_pasid(iommu, sid, pfsid,
> > > pasid, qdep, 0,  
> > 64 - VTD_PAGE_SHIFT);  
> > > +	else
> > > +		qi_flush_dev_iotlb(iommu, sid, pfsid, qdep, 0,
> > > 64 -  
> > VTD_PAGE_SHIFT);  
> > >   }
> > >
> > >   void intel_pasid_tear_down_entry(struct intel_iommu *iommu,
> > > struct  
> > device *dev,  
> > >  

[Jacob Pan]