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Re: [Qemu-ppc] [Qemu-devel] [PATCH] cuda: decrease time delay before rai
From: |
Mark Cave-Ayland |
Subject: |
Re: [Qemu-ppc] [Qemu-devel] [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?
ATB,
Mark.
- [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, 2019/02/12
- Re: [Qemu-ppc] [Qemu-devel] [PATCH] cuda: decrease time delay before raising VIA SR interrupt,
Mark Cave-Ayland <=
- 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