Re: [Qemu-devel] [PATCH v14 3/4] introduce pvevent device to deal with panicked event

[Paolo Bonzini <pbonzini@redhat.com>]

Il 14/03/2013 16:59, Gleb Natapov ha scritto:
> On Thu, Mar 14, 2013 at 04:50:40PM +0100, Paolo Bonzini wrote:
>> Il 14/03/2013 15:23, Gleb Natapov ha scritto:
>>> On Thu, Mar 14, 2013 at 03:05:22PM +0100, Paolo Bonzini wrote:
>>>> Il 14/03/2013 14:56, Gleb Natapov ha scritto:
>>>>> On Thu, Mar 14, 2013 at 02:49:48PM +0100, Paolo Bonzini wrote:
>>>>>> Il 14/03/2013 13:34, Gleb Natapov ha scritto:
>>>>>>>> * it can be an ISA device; the interface is the I/O port and ACPI
>>>>>>>> support is provided just for convenience of the OSPM.  In this case,
>>>>>>>> "-device pvevent" should just add handlers for the port.  The ACPI
>>>>>>>> support is similar to what we do for other on-board ISA devices, for
>>>>>>>> example serial ports (the serial ports use PIIX PCI configuration
>>>>>>>> instead of fw-cfg, but that's a minor detail).  It only needs to work
>>>>>>>> for port 0x505, so the fw-cfg data can be a single yes/no value and only
>>>>>>>> the _STA method needs patching.  See piix4_pm_machine_ready in
>>>>>>>> hw/acpi_piix4.c.
>>>>>>>
>>>>>>> Again I think there is a big difference between well knows device and
>>>>>>> PV devices that we add at random location. And if we make the later
>>>>>>> configurable i.e it may or may not be present and location where it is
>>>>>>> present can be changed then we better not make a guest to do guesses.
>>>>>>
>>>>>> No guesses here on part of the guest, and no probing in the firmware
>>>>>> two.  The same number is hard-coded in QEMU and the DSDT, which go in
>>>>>> pairs anyway, but _not_ in the guest kernel (also thanks to Hu's nice
>>>>>> trick with the methods).
>>>>>
>>>>> That's the problem. The number is not hard coded in QEMU only DSDT.
>>>>
>>>> It is hard-coded where the board creates it, or at least as the default
>>>> value of the qdev property.
>>>
>>> Default value that can be changes is not hard coded.
>>> Why do you allow change in one place, but not the other?
>>
>> I'm just following the model of other ISA devices, I don't think there's
>> any difference in this respect between well-known and pv devices (also
>> because in the end all modern guests will use ACPI to discover even
>> well-known devices).
>>
> We are not there yet :)

Kind of... Windows will hide serial ports that return not-present for
_STA, for example.  Linux will just hide the PNPxxxx path and present it
under /sys/bus/platform instead.

>> The board hardcodes 0x505 for pvpanic just like it hardcodes 0x3f8 for
>> serial ports.
>>
>>>>> If you hard code it in QEMU (make it non configurable) and make device mandatory
>>>>> static DSDT make sense if provided by QEMU.
>>>>
>>>> You cannot make it mandatory due to versioned machine types, but my plan
>>>> would be to make it mandatory on "pc" and "pc-1.5".  For that plan it
>>>> makes sense to have a static DSDT.  Sorry if it was unclear.
>>>
>>> And then you will have to have different DSDT for pre pc-1.5. Dynamic
>>> patching solves exactly that problem.
>>
>> Yes, but it's enough to patch _STA.  Easier in both QEMU and the BIOS.
>>
> Yes, if you do not allow changing IO port patching _STA is enough, but
> if you already patching it is easy to patch both.
> 
>>>>>> I think it's a nice compromise.
>>
>> ^^^ This still holds. :)
> If we would have found a reasonable way to go without patching at all
> then it would have been worthwhile to consider compromises, but if
> patching is inevitable I honestly do not see big difference between
> patching one place or two.

Hmm... can you do something like

        Name(PORT, 0xAAAA)
        OperationRegion(PEOR, SystemIO, PORT, 0x01)
        Field(PEOR, ByteAcc, NoLock, Preserve) {
            PEPT,   8,
        }

? i.e. use a Name inside an OperationRegion?

If so, then we can patch 0xAAAA to zero for not-present and the port for
present and indeed patch a single place.

If we have to patch 0x505 all over the place there's an advantage in
patching _STA only.  But if we can do the above it's a bit cleaner to
use the port for the patched value, indeed.

>> We don't fail machine creation if someone wants to place a serial port
>> at 0x5678.  With ISA it's basically garbage-in, garbage-out, I don't see
>> a reason to make pvpanic special in this respect.
>>
> Fine with me. That was just a suggestion. I thought we had singleton
> qdev flag.

We have no_user, but it's broken and not exactly a match for what you
want here.

Paolo