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 <[email protected]> >
Thanks! Phil.
