On Mon, 19 Sep 2016 23:50:56 +0200
Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> On 19/09/2016 23:36, Alex Williamson wrote:
> > On Tue, 20 Sep 2016 02:05:52 +0530
> > Kirti Wankhede <kwankhede@xxxxxxxxxx> wrote:
> >> 'fb_length':
> >> Read-only file. Mandatory.
> >> Returns <number>{K,M,G}, size of framebuffer.
> >
> > This can't be mandatory, it's only relevant to vGPU devices, vGPUs are
> > just one user of mediated devices. [...]
> >
> >> 'params'
> >> Write-Only file. Optional.
> >> String input. Libvirt would pass the string given in XML file to
> >> this file and then create mdev device. Set empty string to clear params.
> >> For example, set parameter 'frame_rate_limiter=0' to disable frame rate
> >> limiter for performance benchmarking, then create device of type 11. The
> >> device created would have that parameter set by vendor driver.
> >
> > Might as well just call it "magic", there's absolutely no ability to
> > introspect what parameters are allowed or even valid here. Can all
> > parameters be changed dynamically? Do parameter changes apply to
> > existing devices or only future devices? This is a poorly specified
> > interface. I'd prefer this be done via module options on the vendor
> > driver.
>
> Or additional files in the mdev's sysfs directory?
Sure, the vendor driver could certainly support that, it'd be vendor
specific of course.
> >> The parent device would look like:
> >>
> >> <device>
> >> <name>pci_0000_86_00_0</name>
> >> <capability type='pci'>
> >> <domain>0</domain>
> >> <bus>134</bus>
> >> <slot>0</slot>
> >> <function>0</function>
> >
> > This is the parent device address?
>
> (Since this is taken from an email of mine, this is libvirt's XML
> interpretation of the udev database and the contents of sysfs. Try
> "virsh nodedev-dumpxml pci_0000_00_02_0" for an example that is
> unrelated to mdev).
>
> >> 2. Create/destroy mediated device
> >>
> >> With above example, vGPU device XML would look like:
> >>
> >> <device>
> >> <name>my-vgpu</name>
> >> <parent>pci_0000_86_00_0</parent>
> >> <capability type='mdev'>
> >> <type id='11'/>
> >> <group>1</group>
> >> <params>'frame_rate_limiter=0'</params>
> >> </capability>
> >> </device>
> >>
> >> 'type id' is mandatory.
> >
> > I was under the impression that the vendor supplied "name" was the one
> > unique identifier. We're certainly not going to create a registrar to
> > hand out type ids to each vendor, so what makes type id unique?
>
> Why does it have to be unique? It's just parroting what libvirt said in
> the <capability type='mdev'> block of the parent's nodedev-dumpxml.
>
> (Though I guess going by name could be useful as well. Perhaps both
> could be allowed).
Do we not want to be able to migrate a VM to another host and have the
XML mean something? Off-line or on-line, mdev device may support
migration. Thanks,
Alex
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