Messages in this thread | | | Date | Tue, 17 Dec 2002 11:04:51 -0800 (PST) | From | Linus Torvalds <> | Subject | Re: Intel P6 vs P7 system call performance |
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On Tue, 17 Dec 2002, Ulrich Drepper wrote: > > But it will eliminate the problem. Remember: the x86 (unlike x86-64) > has no PC-relative data addressing mode. I.e., in a DSO to find a > memory location with an address I need a base register which isn't > available anymore at the time the call is made.
Actually, I see a more serious problem with the current "syscall" interface: it doesn't allow six-argument system calls AT ALL, since it needed %ebp to keep the stack pointer.
So a six-argument system call _has_ to use "int $0x80" anyway, which to some degree simplifies your problem: you can only use the indirect call approach for things where %ebp will be free for use anyway.
So then you can use %ebp as the indirection, and the code will look something like
games, since that is guaranteed not to be ever used by a system call (it wasn't guaranteed before, but since the sysenter really needs something to hold the stack pointer I made %ebp do that, so there's no way we can ever use %ebp for system calls on x86).
So you _can_ do something like this:
syscall_with_5_args: pushl %ebx pushl %esi pushl %edi pushl %ebp movl syscall_ptr + GOT,%ebp // uses DSO ptr in %ebx or whatever movl $__NR_xxxxxx,%eax movl 20(%esp),%ebx movl 24(%esp),%ecx movl 28(%esp),%edx movl 32(%esp),%esi movl 36(%esp),%edi call *%ebp .. test for errno if needed .. popl %ebp popl %edi popl %esi popl %ebx ret
> You have to assume that all the registers, including %ebp, are used at > the time of the call.
See why this isn't possible right now anyway.
Hmm.. Which system calls have all six parameters? I'll have to see if I can find any way to make those use the new interface too.
In the meantime, I do agree with you that the TLS approach should work too, and might be better. It will allow all six arguments to be used if we just find a good calling conventions (too bad sysenter is such a pig of an instruction, it's really not very well designed since it loses information).
Linus
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