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

From: Philippe Mathieu-Daudé
Subject: Re: [Qemu-devel] [Qemu-ppc] [PATCH] cuda: decrease time delay before raising VIA SR interrupt
Date: Tue, 12 Feb 2019 19:21:24 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0

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>



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