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

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Date:   Fri, 7 Sep 2018 10:43:28 -0700
From:   "Raj, Ashok" <ashok.raj@...el.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        Joerg Roedel <joro@...tes.org>,
        David Woodhouse <dwmw2@...radead.org>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>, "Sun, Yi Y" <yi.y.sun@...el.com>,
        "peterx@...hat.com" <peterx@...hat.com>,
        Jean-Philippe Brucker <jean-philippe.brucker@....com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>
Subject: Re: [PATCH v2 06/12] iommu/vt-d: Add second level page table
 interface

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.

the other optimization was to only do devtlb flushing when you unmap 
since when establish not-present to present there is no need to 
flush devtlb at that point.