[Libguestfs] [PATCH] lib: direct: Remove use of sga

[Daniel P. Berrangé]

On Thu, Sep 09, 2021 at 12:35:12PM +0100, Richard W.M. Jones wrote:
> On Thu, Sep 09, 2021 at 01:11:09PM +0200, Laszlo Ersek wrote:
> > How does libguestfs deal with different QEMU versions / QEMU feature
> > deprecation? Is there some kind of feature detection?
> 
> Kind of, but it's only grepping the output of -help, -device \?,
> and/or looking at QMP output of "query-qmp-schema" and "query-kvm".
> 
> The code is in:
> 
>   https://github.com/libguestfs/libguestfs/blob/master/lib/qemu.c
> 
> As qemu has settled down over time and libguestfs uses only a
> well-established set of features, this kind of feature detection has
> become less and less interesting.  (So quite unlike libvirt where
> they're always trying to catch up with the latest qemu features).  At
> some point we might just say "use qemu >= VERSION" and have done with it.
> 
> > For example, why would it be less good to implement the change as follows:
> > 
> >   if (g->verbose) {
> >     if (guestfs_int_qemu_supports_device(...) {
> >       arg ("-device", "sga");
> >     } else {
> > #if defined(__i386__) || defined(__x86_64__)
> >       arg ("-machine", "graphics=off");
> > #endif
> >     }
> 
> I think in this case it's my understanding that the "new" (from RHEL 7!)
> seabios graphics feature is just better than SGA, and so we might as
> well use it all the time even if SGA is available.  That was my
> understanding from talking to Gerd anyhow.

IIUC, functionally they should be no difference.

The "better" comes from the seabios impl being written in C
while the sgabios impl is 2500 lines of asm :-)


Regards,
Daniel
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