Re: Intel P6 vs P7 system call performance

[Linus Torvalds]

On Tue, 17 Dec 2002, H. Peter Anvin wrote:
>
> This confuses me -- there seems to be no reason this shouldn't work as
> long as %esp == %ebp on sysexit.  The SYSEXIT-trashed GPRs seem like a
> bigger problem.

The thing is, the argument save area == the kernel stack frame. This is
part of the reason why Linux has very fast system calls - there is
absolutely _zero_ extraneous setup. No argument fetching and marshalling,
it's all part of just setting up the regular kernel stack.

So to get the right argument in arg6, the argument _needs_ to be saved in
the %ebp entry on the kernel stack. Which means that on return from the
system call (which may not actually be through a "sysenter" at all, if
signals happen it will go through the generic paths), %ebp will have been
updated as part of the kernel stack unwinding.

Which is ok for a regular fast system call (ebp will get restored
immediately), but it is NOT ok for the system call restart case, since in
that case we want %ebp to contain the old stack pointer, not the sixth
argument.

If we just save the stack pointer value (== the initial %ebx value), the
right thing will get restored, but then system calls will see the stack
pointer value as arg6 - because of the 1:1 relationship between arguments
and stack save.

		Linus