linux-kernel - Re: [RFC PATCH 0/3] VFIO: capability chains

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Date:	Thu, 17 Dec 2015 19:38:37 -0700
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Alexey Kardashevskiy <aik@...abs.ru>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [RFC PATCH 0/3] VFIO: capability chains

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,

Alex
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