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 6:50 PM, Mark Cave-Ayland wrote:
> On 12/02/2019 17:21, Philippe Mathieu-Daudé wrote:
> 
>>>> 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...
> 
> Hmmmm well the existing description already describes the interrupt race in OS 9 so I
> guess the only part missing is the bit about the fast path. How about the revised
> text below for the patch description?
> 
> 
>     cuda: decrease time delay before raising VIA SR interrupt and remove fast path
> 
>     In order to handle a race condition in the MacOS 9 CUDA driver, 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 two things: firstly when booting Linux, MacOS
>     9 and MacOS X the fast path which bypasses the delay is never triggered once the
>     OS kernel is loaded making it effectively useless. Rather than leave this code
>     in place where a guest could potentially enable it by accident and break itself,
>     we might as well just remove it.
> 
>     Secondly the previous reliability issues are no longer present, and this value
>     can be reduced down to 20us with no apparent ill effects. This has the benefit of
>     considerably improving the responsiveness of the ADB keyboard and mouse within
>     the guest.
> 
>     Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
> 

Thanks!

Phil.