linux-kernel - Re: [PATCH v4 04/22] iommu/vt-d: add bind_pasid

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Date:   Fri, 20 Apr 2018 19:25:34 +0100
From:   Jean-Philippe Brucker <Jean-Philippe.Brucker@....com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     Jacob Pan <jacob.jun.pan@...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>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        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>,
        Yi L <yi.l.liu@...ux.intel.com>
Subject: Re: [PATCH v4 04/22] iommu/vt-d: add bind_pasid_table function

On Tue, Apr 17, 2018 at 08:10:47PM +0100, Alex Williamson wrote:
[...]
> > +	/* Assign guest PASID table pointer and size order */
> > +	ctx_lo = (pasidt_binfo->base_ptr & VTD_PAGE_MASK) |
> > +		(pasidt_binfo->pasid_bits - MIN_NR_PASID_BITS);
> 
> Where does this IOMMU API interface define that base_ptr is 4K
> aligned or the format of the PASID table?  Are these all standardized
> or do they vary by host IOMMU?  If they're standards, maybe we could
> note that and the spec which defines them when we declare base_ptr.  If
> they're IOMMU specific then I don't understand how we'll match a user
> provided PASID table to the requirements and format of the host IOMMU.
> Thanks,

On SMMUv3 the minimum alignment for base_ptr is 64 bytes, so a guest
under a vSMMU might pass a pointer that's not aligned on 4k.

Maybe this information could be part of the data passed to userspace
about IOMMU table formats and features? They're not part of this series,
but I think we wanted to communicate IOMMU-specific features via sysfs.

Thanks,
Jean