Re: Is compat_sys_ioctl called when both kernel and userland are 64bit ?

[april <aapril03@gmail.com>]

2009/8/12 Andi Kleen <andi@firstfloor.org>:
> april <aapril03@gmail.com> writes:
>>
>> it seems when a kernel is built to x86_64, whenever a ioctl called
>> ,the call flow will be ia32_syscall -->compat_sys_ioctl->compat
>> ioctl(if provided)
>
> Only for syscalls throught int 0x80 (or 32bit SYSCALL/SYSENTER), which normally
> only come from 32bit processes. In theory 64bit processes
> could use them, but they normally don't.
>

so you mean in 64bit process with a 64bit Linux kernel, system call is
not through int 0x80 ?
I know little about x86_64 arch, so I suppose it still through INT 0x80
can you tell me more about this? or where to get more information abort it?

>> but I have some doubt:
>>
>> My question is:
>> 1.  Is compat_sys_ioctl called when both kernel and userland are 64bit
>> (suppose I provide those compat ioctl functions)?
>
> No.
>
>>      If not, how kernel knows the driver is 32bit or 64bit?
>
> There are no 32bit drivers on a 64bit kernel, all drivers are 64bit.
>
> For 32bit processes running on a 64bit kernel it depends on the
> entry points (int 0x80 or SYSENTER/SYSCALL)
>
>> 2. when using mmap, the 64bit kernel will return a 64bit address, and
>> a userland(32bit) application can only get the lower 32bit,
>> it can work when the memory is not large.
>
> The kernel makes sure to only hand out 32bit addresses to 32bit
> processes.
>
I am using a 64bit kernel ,32bit Ubuntu9.04 which kernel was replaced
by a 64bit one,
and add some compat ioctl to make drm module work(on VIA platform),X
can work well(only 512M ram)
I am not test but guess it will has problem is memory is large (for
example ,over 4G)

> -Andi
>
> --
> ak@linux.intel.com -- Speaking for myself only.
>