[Qemu-devel] seabios serial console vs. sgabios

[Qemu-devel] seabios serial console vs. sgabios

[Gerd Hoffmann]

  Hi,

One problem we have with the serial console support in seabios and
sgabios:  It can happen that both are activated.  We'll go fix that in
qemu, but that'll work for new qemu versions only, not for the old
already released ones.

So I think it would be useful if seabios makes sure sgabios and serial
console are not both active at the same time, i.e. in case we find
"vgaroms/sgabios.bin" being present either turn off builtin serial
console, or don't load sgabios (and possibly also enable builtin serial
console).

Comments?

cheers,
  Gerd

Re: [Qemu-devel] seabios serial console vs. sgabios

[Daniel P. Berrange]

On Thu, Nov 02, 2017 at 05:04:20PM +0100, Gerd Hoffmann wrote:
>   Hi,
> 
> One problem we have with the serial console support in seabios and
> sgabios:  It can happen that both are activated.  We'll go fix that in
> qemu, but that'll work for new qemu versions only, not for the old
> already released ones.
> 
> So I think it would be useful if seabios makes sure sgabios and serial
> console are not both active at the same time, i.e. in case we find
> "vgaroms/sgabios.bin" being present either turn off builtin serial
> console, or don't load sgabios (and possibly also enable builtin serial
> console).

Historically libvirt will always use '-device sgabios' if the user has
requested <bios useserial='yes'/>.  So if that -device arg is given, I
think QEMU must honour it, which implies QEMU must disable Seabios'
own built-in serial console impl in that scenario.

Regards,
Daniel
-- 
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Re: [Qemu-devel] seabios serial console vs. sgabios

[Paolo Bonzini]

On 02/11/2017 17:11, Daniel P. Berrange wrote:
> Historically libvirt will always use '-device sgabios' if the user has
> requested <bios useserial='yes'/>.  So if that -device arg is given, I
> think QEMU must honour it, which implies QEMU must disable Seabios'
> own built-in serial console impl in that scenario.

We should also change libvirt to produce

  -device sga            (if sga supported)
  -machine graphics=off  (if -machine graphics supported)

On old QEMU, -machine graphics=off will be ignored.  On new QEMU and/or
SeaBIOS, we should ensure that specifying both has the right effect.

Paolo

Re: [Qemu-devel] seabios serial console vs. sgabios

[Gerd Hoffmann]

On Thu, 2017-11-02 at 16:11 +0000, Daniel P. Berrange wrote:
> On Thu, Nov 02, 2017 at 05:04:20PM +0100, Gerd Hoffmann wrote:
> >   Hi,
> > 
> > One problem we have with the serial console support in seabios and
> > sgabios:  It can happen that both are activated.  We'll go fix that
> > in
> > qemu, but that'll work for new qemu versions only, not for the old
> > already released ones.
> > 
> > So I think it would be useful if seabios makes sure sgabios and
> > serial
> > console are not both active at the same time, i.e. in case we find
> > "vgaroms/sgabios.bin" being present either turn off builtin serial
> > console, or don't load sgabios (and possibly also enable builtin
> > serial
> > console).
> 
> Historically libvirt will always use '-device sgabios' if the user
> has
> requested <bios useserial='yes'/>.  So if that -device arg is given,
> I
> think QEMU must honour it, which implies QEMU must disable Seabios'
> own built-in serial console impl in that scenario.

Hmm, why?  The user asked for a serial console.  Whenever that is
provided by seabios builtin support or the sgabios rom doesn't really
matter, no?

Also, the question is what seabios should do in case both are active. 
Which I think can happen today with libvirt if you configure a virtual
machine without vga (libvirt passes -machine graphics=off which
activates builtin serial console) and with "<bios useserial='yes'/>"
(enables sgabios rom).

cheers,
  Gerd

Re: [Qemu-devel] seabios serial console vs. sgabios

[Daniel P. Berrange]

On Fri, Nov 03, 2017 at 07:49:03AM +0100, Gerd Hoffmann wrote:
> On Thu, 2017-11-02 at 16:11 +0000, Daniel P. Berrange wrote:
> > On Thu, Nov 02, 2017 at 05:04:20PM +0100, Gerd Hoffmann wrote:
> > >   Hi,
> > > 
> > > One problem we have with the serial console support in seabios and
> > > sgabios:  It can happen that both are activated.  We'll go fix that
> > > in
> > > qemu, but that'll work for new qemu versions only, not for the old
> > > already released ones.
> > > 
> > > So I think it would be useful if seabios makes sure sgabios and
> > > serial
> > > console are not both active at the same time, i.e. in case we find
> > > "vgaroms/sgabios.bin" being present either turn off builtin serial
> > > console, or don't load sgabios (and possibly also enable builtin
> > > serial
> > > console).
> > 
> > Historically libvirt will always use '-device sgabios' if the user
> > has
> > requested <bios useserial='yes'/>.  So if that -device arg is given,
> > I
> > think QEMU must honour it, which implies QEMU must disable Seabios'
> > own built-in serial console impl in that scenario.
> 
> Hmm, why?  The user asked for a serial console.  Whenever that is
> provided by seabios builtin support or the sgabios rom doesn't really
> matter, no?

Surely it has ABI implications for the guest if we stop putting the
sgabios option ROM into memory - changing layout of ROMS

> Also, the question is what seabios should do in case both are active. 
> Which I think can happen today with libvirt if you configure a virtual
> machine without vga (libvirt passes -machine graphics=off which
> activates builtin serial console) and with "<bios useserial='yes'/>"
> (enables sgabios rom).

Libvirt doesn't use  '-machine graphics=off' AFAIK, only '-nographic'. 

Regards,
Daniel
-- 
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Re: [Qemu-devel] seabios serial console vs. sgabios

[Gerd Hoffmann]

  Hi,

> > Hmm, why?  The user asked for a serial console.  Whenever that is
> > provided by seabios builtin support or the sgabios rom doesn't
> > really
> > matter, no?
> 
> Surely it has ABI implications for the guest if we stop putting the
> sgabios option ROM into memory - changing layout of ROMS

Because of that changing -device sga behavior on the qemu side must be
tied to the qemu machine type.

> > Also, the question is what seabios should do in case both are
> > active. 
> > Which I think can happen today with libvirt if you configure a
> > virtual
> > machine without vga (libvirt passes -machine graphics=off which
> > activates builtin serial console) and with "<bios
> > useserial='yes'/>"
> > (enables sgabios rom).
> 
> Libvirt doesn't use  '-machine graphics=off' AFAIK, only '-
> nographic'.

I think that is pretty much the same.  Setting "graphichs=off" is one
of the effects of passing -nographic, and the other effects (like
setting up default serial + monitor in a different way) don't happen
due to libvirt also using -nodefaults.

cheers,
  Gerd

Re: [Qemu-devel] seabios serial console vs. sgabios

[Markus Armbruster]

Gerd Hoffmann <kraxel@redhat.com> writes:

[...]
>> Libvirt doesn't use  '-machine graphics=off' AFAIK, only '-
>> nographic'.
>
> I think that is pretty much the same.  Setting "graphichs=off" is one
> of the effects of passing -nographic, and the other effects (like
> setting up default serial + monitor in a different way) don't happen
> due to libvirt also using -nodefaults.

-nographic is a legacy / convenience option these days.  I'd recommend
management tools use -machine graphics=off instead.

Re: [Qemu-devel] [SeaBIOS] seabios serial console vs. sgabios

[Kevin O'Connor]

On Thu, Nov 02, 2017 at 05:04:20PM +0100, Gerd Hoffmann wrote:
>   Hi,
> 
> One problem we have with the serial console support in seabios and
> sgabios:  It can happen that both are activated.  We'll go fix that in
> qemu, but that'll work for new qemu versions only, not for the old
> already released ones.
> 
> So I think it would be useful if seabios makes sure sgabios and serial
> console are not both active at the same time, i.e. in case we find
> "vgaroms/sgabios.bin" being present either turn off builtin serial
> console, or don't load sgabios (and possibly also enable builtin serial
> console).

I'm not sure on this one.  If qemu tells seabios to load a rom, I
think it really should do that.

If future versions of qemu will be changed to not request sgabios when
sercon is enabled, then maybe the issue isn't large enough to worry
about (it only affects no-graphics, new seabios, old qemu).  If that's
not safe, perhaps we should revisit how seabios enables sercon so
there is no confusion.

-Kevin