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

[Laszlo Ersek <lersek@redhat.com>]

On 06/17/13 11:57, 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 <mst@redhat.com>
>>>>> ---
>>>>>  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.

Yes, I think that's exactly what happens *if* at domain creation time
the Xen userspace utilities start qemu with such a machine model that
sets "has_pvpanic" to false. I'd only like to have confirmation that the
leading comment on pc_init_pci_no_kvmclock() is up-to-date and we can
trust this code never to run on Xen.

Actually, we can figure out later, if/when it breaks under Xen. It
shouldn't be hard to fix.

series
Reviewed-by: Laszlo Ersek <lersek@redhat.com>