Subject: Re: [RFC PATCH 0/3] VFIO: capability chains
To: Alex Williamson <alex.williamson@redhat.com>
References: <20151123202614.18252.41590.stgit@gimli.home>
 <567369FD.4000100@ozlabs.ru> <1450406317.2674.160.camel@redhat.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org
From: Alexey Kardashevskiy <aik@ozlabs.ru>
Message-ID: <56738CD6.7000105@ozlabs.ru>
Date: Fri, 18 Dec 2015 15:34:30 +1100
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On 12/18/2015 01:38 PM, Alex Williamson wrote:
> On Fri, 2015-12-18 at 13:05 +1100, Alexey Kardashevskiy wrote:
>> On 11/24/2015 07:43 AM, Alex Williamson wrote:
>>> Please see the commit log and comments in patch 1 for a general
>>> explanation of the problems that this series tries to address.  The
>>> general problem is that we have several cases where we want to
>>> expose
>>> variable sized information to the user, whether it's sparse mmaps
>>> for
>>> a region, as implemented here, or DMA mapping ranges of an IOMMU,
>>> or
>>> reserved MSI mapping ranges, etc.  Extending data structures is
>>> hard;
>>> extending them to report variable sized data is really hard.  After
>>> considering several options, I think the best approach is to copy
>>> how
>>> PCI does capabilities.  This allows the ioctl to only expose the
>>> capabilities that are relevant for them, avoids data structures
>>> that
>>> are too complicated to parse, and avoids creating a new ioctl each
>>> time we think of something else that we'd like to report.  This
>>> method
>>> also doesn't preclude extensions to the fixed structure since the
>>> offset of these capabilities is entirely dynamic.
>>>
>>> Comments welcome, I'll also follow-up to the QEMU and KVM lists
>>> with
>>> an RFC making use of this for mmaps skipping over the MSI-X table.
>>> Thanks,
>>
>> Out of curiosity - could this information be exposed to the userspace
>> via
>> /sys/bus/pci/devices/xxxx:xx:xx:x/vfio_xxxx? It seems not to change
>> after
>> vfio_pci driver is bound to a device.
>
> For what purpose?  vfio doesn't have a sysfs interface, why start one?
> Thanks,

well, it could simplify debugging a bit if this information was available 
from the userspace without programming a test tool doing some ioctl()'s. 
Not a big deal though...


-- 
Alexey
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