linux-kernel - Re: [PATCH v4 05/22] iommu: introduce iommu invalidate API function

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Date:   Tue, 8 May 2018 11:35:00 +0100
From:   Jean-Philippe Brucker <jean-philippe.brucker@....com>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     "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>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Rafael Wysocki <rafael.j.wysocki@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        Jean Delvare <khali@...ux-fr.org>,
        Christoph Hellwig <hch@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        "Liu, Yi L" <yi.l.liu@...ux.intel.com>,
        "Liu@...rya.localdomain" <Liu@...rya.localdomain>
Subject: Re: [PATCH v4 05/22] iommu: introduce iommu invalidate API function

Hi Jacob,

Looks mostly good to me, I just have a couple more comments

On 04/05/18 19:07, Jacob Pan wrote:
> Now the passdown invalidation granularities look like:
> (sorted by coarseness), will send out in v5 patchset soon if no issues.
> 
> /**
>  * enum iommu_inv_granularity - Generic invalidation granularity
>  *
>  * @IOMMU_INV_GRANU_DOMAIN:		Device context cache associated with a
>  *					domain ID.
>  * @IOMMU_INV_GRANU_DEVICE:		Device context cache associated with a
>  *					device ID
>  * @IOMMU_INV_GRANU_DOMAIN_ALL_PASID:	TLB entries or PASID caches of all
>  *					PASIDs associated with a domain ID
>  * @IOMMU_INV_GRANU_PASID_SEL:		TLB entries or PASID cache associated
>  *					with a PASID and a domain
>  * @IOMMU_INV_GRANU_PAGE_PASID:		TLB entries of selected page range
>  *					within a PASID
>  *
>  * When an invalidation request is passed down to IOMMU to flush translation
>  * caches, it may carry different granularity levels, which can be specific
>  * to certain types of translation caches. For an example, PASID selective
>  * granularity is only applicable PASID cache and IOTLB invalidation but for
>  * device context caches.

Should it be "PASID selective granularity is only applicable to PASID
cache and IOTLB but not device context caches"?

>  * This enum is a collection of granularities for all types of translation
>  * caches. The idea is to make it easy for IOMMU model specific driver to
>  * convert from generic to model specific value. Not all combinations between
>  * translation caches and granularity levels are valid. Each IOMMU driver
>  * can enforce check based on its own conversion table. The conversion is
>  * based on 2D look-up with inputs as follows:
>  * - translation cache types
>  * - granularity
>  * No global granularity is allowed in that passdown invalidation for an
>  * assigned device should only impact the device or domain itself.

That last sentence is a bit confusing, because "global granularity"
might also refer to the "global" TLB flag which is allowed. In my
opinion you can leave this rationale out, I doubt userspace will ever
demand a mechanism for global invalidation.

>  *
>  *             type |   DTLB    |    TLB    |   PASID   |  CONTEXT
>  *  granule         |           |           |           |
>  * -----------------+-----------+-----------+-----------+-----------
>  *  DOMAIN          |           |           |           |     Y
>  *  DEVICE          |           |           |           |     Y

I can't really see a use-case for DOMAIN and DEVICE. It might make more
sense to keep only DN_ALL_PASID, which would then also invalidate the
device context cache. But since they will be very rare events, factoring
them doesn't seem important.

>  *  DN_ALL_PASID    |   Y       |   Y       |   Y       |
>  *  PASID_SEL       |   Y       |   Y       |   Y       |
>  *  PAGE_PASID      |           |   Y       |           |

Why not allow PAGE_PASID+DTLB? We need a way to invalidate individual
DTLB entries

Thanks,
Jean