Re: [Qemu-devel] [Qemu-ppc] [PATCH] cuda: decrease time delay before raising VIA SR interrupt

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 2/12/19 5:51 PM, Mark Cave-Ayland wrote:
> On 12/02/2019 11:03, BALATON Zoltan wrote:
> 
>> On Tue, 12 Feb 2019, Mark Cave-Ayland wrote:
>>> On 11/02/2019 23:35, Philippe Mathieu-Daudé wrote:
>>>
>>>> Hi Mark,
>>>>
>>>> On 2/10/19 6:44 PM, Mark Cave-Ayland wrote:
>>>>> In order to handle a race condition in MacOS 9, a delay was introduced when
>>>>> raising the VIA SR interrupt inspired by similar code in MacOnLinux.
>>>>>
>>>>> During original testing of the MacOS 9 patches it was found that the 30us
>>>>> delay used in MacOnLinux did not work reliably within QEMU, and a value of
>>>>> 300us was required to function correctly.
>>>>>
>>>>> Recent experiments have shown that the previous reliability issues are no
>>>>> longer present, and this value can be reduced down to 20us with no apparent
>>>>> ill effects in my local tests. This has the benefit of considerably improving
>>>>> the responsiveness of the ADB keyboard and mouse with the guest.
>>>>>
>>>>> Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
>>>>> ---
>>>>>  hw/misc/macio/cuda.c | 11 +----------
>>>>>  1 file changed, 1 insertion(+), 10 deletions(-)
>>>>>
>>>>> diff --git a/hw/misc/macio/cuda.c b/hw/misc/macio/cuda.c
>>>>> index c4f7a2f39b..3febacdd1e 100644
>>>>> --- a/hw/misc/macio/cuda.c
>>>>> +++ b/hw/misc/macio/cuda.c
>>>>> @@ -97,17 +97,8 @@ static void cuda_set_sr_int(void *opaque)
>>>>>
>>>>>  static void cuda_delay_set_sr_int(CUDAState *s)
>>>>>  {
>>>>> -    MOS6522CUDAState *mcs = &s->mos6522_cuda;
>>>>> -    MOS6522State *ms = MOS6522(mcs);
>>>>> -    MOS6522DeviceClass *mdc = MOS6522_DEVICE_GET_CLASS(ms);
>>>>>      int64_t expire;
>>>>>
>>>>> -    if (ms->dirb == 0xff || s->sr_delay_ns == 0) {
>>>>> -        /* Disabled or not in Mac OS, fire the IRQ directly */
>>>>> -        mdc->set_sr_int(ms);
>>>>> -        return;
>>>>> -    }
>>>>
>>>> The change of sr_delay_ns below is well explained, but I don't
>>>> understand why you remove the previous if().
>>>
>>> IIRC it was a hack by Alex to try and restrict the delay on the interrupt just to
>>> MacOS instead of Linux, but with the reduced value it doesn't really matter any more.
>>
>> If this delay is to prevent a bug which only happens in MacOS then that's the hack
>> not the normal code path to run without the delay that you've just removed. So maybe
>> this should be kept if possible to avoid unecessary delays for other guests.
>> (Although if this only affects mac99,via=cuda but not mac99,via=pmu then I don't care
>> much as long as pmu works.)
> 
> Well the reality is that the detection above doesn't actually seem to work anyway -
> at least a quick boot test with Linux, MacOS X and MacOS 9 with a printf() added into
> the if() shows nothing firing once the kernel takes over. So the slow path with the
> delay included was always being taken within the OS anyway.
> 
> And indeed, the code doesn't affect pmu so you won't see any difference there.
> 
>>> As a plus it also prevents a guest OS from accidentally triggering the hack whilst
>>> programming the VIA port.
>>
>> That may be a problem though. What's the issue exactly? Why is the delay needed in
>> the first place?
> 
> It's some kind of racy polling with OS 9 (I wasn't involved in the technical details,
> sorry) which causes OS 9 to hang on boot if the delay isn't present. And even better
> the slow path that was previously always being taken has now been reduced from 300us
> to 30us so whichever way you look at it, having this patch applied is a win.

Can you write a paragraph about this, that David can amend to your
patch? That would stop worrying me about looking at this patch in
various months...

Thanks!

Phil.