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Re: [Qemu-ppc] [Qemu-devel] [PATCH] cuda: decrease time delay before rai
From: |
Philippe Mathieu-Daudé |
Subject: |
Re: [Qemu-ppc] [Qemu-devel] [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>
>
Thanks!
Phil.
- [Qemu-ppc] [PATCH] cuda: decrease time delay before raising VIA SR interrupt, Mark Cave-Ayland, 2019/02/10
- Re: [Qemu-ppc] [PATCH] cuda: decrease time delay before raising VIA SR interrupt, David Gibson, 2019/02/10
- Re: [Qemu-ppc] [Qemu-devel] [PATCH] cuda: decrease time delay before raising VIA SR interrupt, Philippe Mathieu-Daudé, 2019/02/11
- Re: [Qemu-ppc] [Qemu-devel] [PATCH] cuda: decrease time delay before raising VIA SR interrupt, Mark Cave-Ayland, 2019/02/12
- Re: [Qemu-ppc] [Qemu-devel] [PATCH] cuda: decrease time delay before raising VIA SR interrupt, BALATON Zoltan, 2019/02/12
- Re: [Qemu-ppc] [Qemu-devel] [PATCH] cuda: decrease time delay before raising VIA SR interrupt, Mark Cave-Ayland, 2019/02/12
- Re: [Qemu-ppc] [Qemu-devel] [PATCH] cuda: decrease time delay before raising VIA SR interrupt, Philippe Mathieu-Daudé, 2019/02/12
- Re: [Qemu-ppc] [Qemu-devel] [PATCH] cuda: decrease time delay before raising VIA SR interrupt, Mark Cave-Ayland, 2019/02/12
- Re: [Qemu-ppc] [Qemu-devel] [PATCH] cuda: decrease time delay before raising VIA SR interrupt,
Philippe Mathieu-Daudé <=
- Re: [Qemu-ppc] [Qemu-devel] [PATCH] cuda: decrease time delay before raising VIA SR interrupt, Mark Cave-Ayland, 2019/02/12
- Re: [Qemu-ppc] [Qemu-devel] [PATCH] cuda: decrease time delay before raising VIA SR interrupt, David Gibson, 2019/02/12
- Re: [Qemu-ppc] [Qemu-devel] [PATCH] cuda: decrease time delay before raising VIA SR interrupt, Mark Cave-Ayland, 2019/02/13