linux-kernel - RE: [PATCH v2 06/12] iommu/vt-d: Add second level page table interface

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Message-ID: <AADFC41AFE54684AB9EE6CBC0274A5D191301901@SHSMSX101.ccr.corp.intel.com>
Date:   Thu, 13 Sep 2018 05:52:32 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     "Raj, Ashok" <ashok.raj@...el.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>
CC:     Jean-Philippe Brucker <jean-philippe.brucker@....com>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Sun, Yi Y" <yi.y.sun@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        David Woodhouse <dwmw2@...radead.org>
Subject: RE: [PATCH v2 06/12] iommu/vt-d: Add second level page table
 interface

> From: Raj, Ashok
> Sent: Saturday, September 8, 2018 1:43 AM
> 
> On Fri, Sep 07, 2018 at 10:47:11AM +0800, Lu Baolu wrote:
> >
> > >>+
> > >>+	intel_pasid_clear_entry(dev, pasid);
> > >>+
> > >>+	if (!ecap_coherent(iommu->ecap)) {
> > >>+		pte = intel_pasid_get_entry(dev, pasid);
> > >>+		clflush_cache_range(pte, sizeof(*pte));
> > >>+	}
> > >>+
> > >>+	pasid_based_pasid_cache_invalidation(iommu, did, pasid);
> > >>+	pasid_based_iotlb_cache_invalidation(iommu, did, pasid);
> > >>+
> > >>+	/* Device IOTLB doesn't need to be flushed in caching mode. */
> > >>+	if (!cap_caching_mode(iommu->cap))
> > >>+		pasid_based_dev_iotlb_cache_invalidation(iommu, dev,
> > >>pasid);
> > >
> > >can you elaborate, or point to any spec reference?
> > >
> >
> > In the driver, device iotlb doesn't get flushed in caching mode. I just
> > follow what have been done there.
> >
> > It also makes sense to me since only the bare metal host needs to
> > consider whether and how to flush the device iotlb.
> >
> 
> DavidW might remember, i think the idea was to help with cost
> of virtualization, we can avoid taking 2 exits vs handling
> it directly when we do iotlb flushing instead.
> 

OK, performance-wise it makes sense. though strictly speaking it
doesn't follow spec...

Thanks
Kevin