Re: [Qemu-devel] [Qemu-ppc] [PATCH] cuda: decrease time delay before raising VIA SR interrupt

[David Gibson <david@gibson.dropbear.id.au>]

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On Tue, Feb 12, 2019 at 08:01:22PM +0000, Mark Cave-Ayland wrote:
> 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 <mark.cave-ayland@ilande.co.uk>
> >>
> > 
> > Thanks!
> > 
> > Phil.
> 
> No worries. David, are you able to update the commit message in your ppc-for-4.0
> branch accordingly?

Done.

> 
> 
> ATB,
> 
> Mark.
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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