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Re: [Qemu-devel] [Qemu-ppc] [PATCH] cuda: decrease time delay before rai

From: Mark Cave-Ayland
Subject: Re: [Qemu-devel] [Qemu-ppc] [PATCH] cuda: decrease time delay before raising VIA SR interrupt
Date: Tue, 12 Feb 2019 20:01:22 +0000
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0

On 12/02/2019 18:21, Philippe Mathieu-Daudé wrote:

> 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 <address@hidden>
> Thanks!
> Phil.

No worries. David, are you able to update the commit message in your ppc-for-4.0
branch accordingly?



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