linux-kernel - Re: [EXT] Re: [PATCH v5] iommu/virtio: Use page size bitmap supported by endpoint

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Message-ID: <20200513104537.GB214296@myrica>
Date:   Wed, 13 May 2020 12:45:37 +0200
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Bharat Bhushan <bbhushan2@...vell.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "eric.auger.pro@...il.com" <eric.auger.pro@...il.com>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>
Subject: Re: [EXT] Re: [PATCH v5] iommu/virtio: Use page size bitmap
 supported by endpoint

On Wed, May 13, 2020 at 09:15:22AM +0000, Bharat Bhushan wrote:
> Hi Jean,
> 
> > -----Original Message-----
> > From: Michael S. Tsirkin <mst@...hat.com>
> > Sent: Wednesday, May 6, 2020 5:53 AM
> > To: Bharat Bhushan <bbhushan2@...vell.com>
> > Cc: jean-philippe@...aro.org; joro@...tes.org; jasowang@...hat.com;
> > virtualization@...ts.linux-foundation.org; iommu@...ts.linux-foundation.org;
> > linux-kernel@...r.kernel.org; eric.auger.pro@...il.com; eric.auger@...hat.com
> > Subject: [EXT] Re: [PATCH v5] iommu/virtio: Use page size bitmap supported by
> > endpoint
[...]
> > > +struct virtio_iommu_probe_pgsize_mask {
> > > +	struct virtio_iommu_probe_property	head;
> > > +	__u8					reserved[4];
> > > +	__le64					pgsize_bitmap;
> > > +};
> > > +
> > 
> > This is UAPI. Document the format of pgsize_bitmap please.
> 
> I do not see uapi documentation in "Documentation" folder of other data struct in this file, am I missing something?

I'm not the one requesting this change, but I'm guessing you should add a
comment in this header, above pgsize_bitmap (which should actually be
called page_size_mask to follow the spec). I guess I'd add:

/* Same format as virtio_iommu_config::page_size_mask */

for this field, and then maybe change the comment of virtio_iommu_config,
currently "/* Supported page sizes */", to something more precise such as:

/*
 * Bitmap of supported page sizes. The least significant bit indicates the
 * smallest granularity and the other bits are hints indicating optimal
 * block sizes.
 */

But I don't know how much should be spelled out here, since those details
are available in the spec.

Thanks,
Jean