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Re: [libvirt] [PATCH 1/2] conf: add debugcon-isa chardev guest interface



On Mon, Jan 28, 2019 at 11:24:38AM +0100, Andrea Bolognani wrote:
> On Fri, 2019-01-25 at 14:06 +0000, Nikolay Shirokovskiy wrote:
> > On 25.01.2019 16:37, Ján Tomko wrote:
> > > On Thu, Jan 24, 2019 at 05:28:59PM +0300, Nikolay Shirokovskiy wrote:
> > > > This interface can be used for example by firmware to print
> > > > debug messages. Here is domain xml example:
> > > > 
> > > >    <channel type='file'>
> > > >      <source path='/var/log/libvirt/qemu/VM.firmware.log'/>
> > > >      <target type='debugcon-isa'/>
> > > 
> > > IMO 'debugcon' would be enough.
> > 
> > I followed serial chardev xml. It uses pci-serial, usb-serial etc
> > for target type. I think idea is to make it possible to specify
> > type when address is not specified (if it is then one can use 
> > address type to distinguish). Of course we have only isa-debugcon
> > device yet. However I agree that for channel configuration having
> > enumeration like spicevm, guesfw, debugcon-isa looks like mixing
> > types and subtypes.
> 
> Should this be a <channel> at all?
> 
> We generally use <serial> for native emulated devices such as
> isa-serial, which isa-debugcon seems very closely related to...

<channel> is aiming to expose application specific guest data channels.

<serial> is aiming to expose general purpose guest data channels.

To me isa-debugcon is an application specific data channels, so
<channel> feels a better fit.

> Perhaps the best way to hook this up would be
> 
>   <serial type='pty'>
>     <target type='isa-serial'>

IMHO using 'isa-serial' would be misleading for applications which
know about <serial> ports today and may well cause unexpected side
effects, especially wrt to the <console> mapping of the first <serial>
port.

>       <model name='isa-debugcon'/>
>     </target>
>   </serial>
> 
> CC'ing Pavel as we looked into this mess together the last time
> around, so he might have an opinion on the subject :)

Regards,
Daniel
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