FEXCore: adds support for compat input prctl #4106
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The size of the input_event struct differs between 32 bits applications and 64 bits applications. To deal with this, the kernel implements a compat variant for the input syscalls, but it's only enabled for 32 bit processes. In libkrunfw we're introducing a prctl that enables a 64 bit process to request the kernel to enable the compat variant for the input syscalls. This commit makes use of that interface for enabling/disabling the compat input variant as required. The visible effect is that input devices such as gamepads work properly on emulated 32 bit applications. Signed-off-by: Sergio Lopez <[email protected]>
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This one needs to be paired with: slp/linux@897560f Does this kind of feature require a |
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Short term this is probably a fine workaround. Long term I think it'll have problems when we do more 32-bit thunking and then 32-bit and 64-bit libraries are both reading gamepad inputs, but since that's a non-issue for today, it's not a real concern yet.
FEXConfig shouldn't matter here, it's a runtime detectable option and would always want it currently. I assume your kernel code is having processes inherit the prctl value being set, which is why it happens in all processes. Seems reasonable to me. |
I don't expect Linux FEX to ever thunk anything that would read gamepads on 32-bit. Even thunking things like SDL on 64-bit doesn't seem to vibe with how we've approached thunking in FEX. I know box does it but they make compatibility tradeoffs in doing so that I'd rather FEX not. |
Yes, I also consider this a temporary fix.
Originally I was clearing the value on fork(), but I've noticed trouble in some 32 bits apps. I didn't dig deep, but my guess is that 32 bits apps may fork without going to through the FEXLoader's main (which does makes sense). Keeping the value around and disabling it for 64 bit apps makes everything work as expected. |
FTR libSDL is definitely one of the better-behaving APIs. This should be straightforward to forward to host now that we have a better approach to forwarding windowing system use. |
The size of the input_event struct differs between 32 bits applications and 64 bits applications. To deal with this, the kernel implements a compat variant for the input syscalls, but it's only enabled for 32 bit processes.
In libkrunfw we're introducing a prctl that enables a 64 bit process to request the kernel to enable the compat variant for the input syscalls. This commit makes use of that interface for enabling/disabling the compat input variant as required.
The visible effect is that input devices such as gamepads work properly on emulated 32 bit applications.