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Re: [Qemu-devel] [PATCH v3 5/7] hmp: Add info commands for preconfig

From: Dr. David Alan Gilbert
Subject: Re: [Qemu-devel] [PATCH v3 5/7] hmp: Add info commands for preconfig
Date: Tue, 12 Jun 2018 13:52:54 +0100
User-agent: Mutt/1.10.0 (2018-05-17)

* Markus Armbruster (address@hidden) wrote:
> Gerd Hoffmann <address@hidden> writes:
> >   Hi,
> >
> >> > Now let's review the three commands:
> >> > 
> >> > * Gerd, why does "info usbhost" have no QMP equivalent?
> >
> > Works only when running qemu directly, in the libvirt sandbox qemu
> > hasn't the permissions needed to scan the host usb bus so that would be
> > rather pointless ...
> I don't think this meets either of the two criteria:
> * I meets "makes no sense in QMP" only if QMP implies "can't scan host
>   USB bus".  Libvirt implies it, but QMP doesn't imply libvirt; it's its
>   most important, not its sole user.
> * It meets "of use only for human users" only if we're convinced it's of
>   no use to programs.
>   To avoid speculation and endless arguments about what could or could
>   not be of use, we've always stuck to "when in doubt, assume it could
>   be of use".
>   "Libvirt can't use it" falls short.
>   "Any management application worth anything would deny QEMU the
>   capability to scan the USB host bus, and thus wouldn't be able to use
>   it" is exactly the argument we intended to avoid.
> QMP falling short of completeness in relatively unimportant ways like
> this one isn't exactly terrible.  The most serious effect is probably
> serving as a bad example that leads to further arguments like this one.
> These are well worth avoiding, though.

  a) This is a separate discussion; info usbhost has been there for many
years;  this patch set doesn't change that.
  b) From HMP, if someone wants to add a command like 'info usbhost' to
make their debugging of USB easy, then HMP is all for that and there's
no way I'm going to require QMP implementations for a debug command.


Dr. David Alan Gilbert / address@hidden / Manchester, UK

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