Re: [Qemu-devel] [PATCH 1/2] pvpanic: initialization cleanup

[Stefano Stabellini]

On Mon, 17 Jun 2013, Michael S. Tsirkin wrote:
> On Mon, Jun 17, 2013 at 11:35:00AM +0200, Laszlo Ersek wrote:
> > On 06/17/13 11:19, Michael S. Tsirkin wrote:
> > > On Mon, Jun 17, 2013 at 09:56:56AM +0200, Laszlo Ersek wrote:
> > >> On 06/16/13 22:59, Michael S. Tsirkin wrote:
> > >>> Avoid use of static variables: PC systems initialize pvpanic device
> > >>> through pvpanic_init, so we can simply create the fw_cfg file at that
> > >>> point.  Others don't use fw_cfg at all.  This also makes it possible to
> > >>> assert if fw_cfg is not there rather than skipping the device silently.
> > >>>
> > >>> Signed-off-by: Michael S. Tsirkin <address@hidden>
> > >>> ---
> > >>>  hw/misc/pvpanic.c | 23 ++++++++++-------------
> > >>>  1 file changed, 10 insertions(+), 13 deletions(-)
> > >>>
> > >>> diff --git a/hw/misc/pvpanic.c b/hw/misc/pvpanic.c
> > >>> index 060099b..9ed9897 100644
> > >>> --- a/hw/misc/pvpanic.c
> > >>> +++ b/hw/misc/pvpanic.c
> > >>> @@ -97,25 +97,22 @@ static void pvpanic_isa_realizefn(DeviceState *dev, 
> > >>> Error **errp)
> > >>>  {
> > >>>      ISADevice *d = ISA_DEVICE(dev);
> > >>>      PVPanicState *s = ISA_PVPANIC_DEVICE(dev);
> > >>> -    static bool port_configured;
> > >>> -    FWCfgState *fw_cfg;
> > >>>  
> > >>>      isa_register_ioport(d, &s->io, s->ioport);
> > >>> -
> > >>> -    if (!port_configured) {
> > >>> -        fw_cfg = fw_cfg_find();
> > >>> -        if (fw_cfg) {
> > >>> -            fw_cfg_add_file(fw_cfg, "etc/pvpanic-port",
> > >>> -                            g_memdup(&s->ioport, sizeof(s->ioport)),
> > >>> -                            sizeof(s->ioport));
> > >>> -            port_configured = true;
> > >>> -        }
> > >>> -    }
> > >>>  }
> > >>>  
> > >>>  int pvpanic_init(ISABus *bus)
> > >>>  {
> > >>> -    isa_create_simple(bus, TYPE_ISA_PVPANIC_DEVICE);
> > >>> +    ISADevice *dev = isa_create_simple(bus, TYPE_ISA_PVPANIC_DEVICE);
> > >>> +    PVPanicState *s = ISA_PVPANIC_DEVICE(dev);
> > >>> +    FWCfgState *fw_cfg = fw_cfg_find();
> > >>> +
> > >>> +    assert(fw_cfg);
> > >>
> > >> Won't the assert fire if:
> > >>
> > >>   xen_enabled() &&
> > >>   machine != "pc-0.10" && machine != "pc-0.11" &&
> > >>   machine != "pc-0.12" && machine != "pc-0.13" &&
> > >>   machine != "pc-q35-1.4"
> > >>
> > >> Because under the above condition "has_pvpanic" remains "true", but
> > >> fw_cfg is not initialized.
> > >>
> > >> (pc_init_pci_no_kvmclock() in "hw/i386/pc_piix.c" sets "has_pvpanic" to
> > >> "false", and claims to be "reused by xenfv", so the above condition may
> > >> be constant false.)
> > > 
> > > That's what I think - if user wants pvpanic to work, fw cfg is required 
> > > ATM.
> > 
> > What I have in mind is the following: suppose xen is enabled and qemu is
> > started with -M pc-i440fx-1.5.
> > 
> > Before the patch, the pvpanic device didn't work, but qemu didn't crash
> > either. After the patch, the assert() is triggered at startup.
> > 
> > Of course, if starting qemu for xen with "-M pc-i440fx-1.5" is *already*
> > broken (for other, maybe more serious, reasons), ie. PEBKAC, then the
> > patch is correct. But I can't evaluate that condition to constant false,
> > and suppose that it's a possible configuration, under which qemu would
> > now start with an assertion failure.
> > 
> > Can someone with Xen knowledge chime in? CC'ing Stefano.
> > 
> > Laszlo
> 
> A sane alternative is to avoid creating the pvpanic device.
> Not as easy to debug as an assert, but at least
> guest does not get reserved ports which said guest
> has no way to discover.

Yeah, that's pretty bad. Even though at the moment the Xen tools would
always create QEMU with -M xenfv, I would like to be able to use other
QEMUMachines too in the future.