Subject: Re: [PATCH] vfio: Simplify capability helper
To: Peter Xu <peterx@redhat.com>,
        Alex Williamson <alex.williamson@redhat.com>
References: <20171212195850.13691.40496.stgit@gimli.home>
 <20171213064936.GC7780@xz-mi>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        intel-gvt-dev@lists.freedesktop.org, aik@ozlabs.ru,
        kwankhede@nvidia.com, zhenyuw@linux.intel.com, zhi.a.wang@intel.com
From: Auger Eric <eric.auger@redhat.com>
Message-ID: <0bb4e51a-6fd2-e946-8927-b8d9fe491b07@redhat.com>
Date: Wed, 13 Dec 2017 16:04:48 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101
 Thunderbird/45.4.0
MIME-Version: 1.0
In-Reply-To: <20171213064936.GC7780@xz-mi>
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1372
Lines: 36

Hi Peter,

On 13/12/17 07:49, Peter Xu wrote:
> On Tue, Dec 12, 2017 at 12:59:39PM -0700, Alex Williamson wrote:
>> The vfio_info_add_capability() helper requires the caller to pass a
>> capability ID, which it then uses to fill in header fields, assuming
>> hard coded versions.  This makes for an awkward and rigid interface.
>> The only thing we want this helper to do is allocate sufficient
>> space in the caps buffer and chain this capability into the list.
>> Reduce it to that simple task.
>>
>> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> 
> Reviewed-by: Peter Xu <peterx@redhat.com>
> 
> Though during review I had a question related to the function
> msix_sparse_mmap_cap(): Is it possible that one PCI device BAR is very
> small (e.g., 4K) that only contains the MSI-X table (and another small
> PBA area)?  If so, should we just mark that region unmappable instead
> of setting vfio_region_info_cap_sparse_mmap.nr_areas to 1 in
> msix_sparse_mmap_cap()?
> 
> 	/* If MSI-X table is aligned to the start or end, only one area */
> 	if (((vdev->msix_offset & PAGE_MASK) == 0) ||
> 	    (PAGE_ALIGN(vdev->msix_offset + vdev->msix_size) >= end))
> 		nr_areas = 1;
> 
> Thanks,
> 
if I understand the code correctly, if the MSI-X table exactly matches
the BAR, a sparse mmap region is reported with offset/size = 0. Is that
correct?

Thanks

Eric